Will a cheap £2 optical splitter work ?

Why not buy one and find out.
If your question concerns some Sonos equipment then lots and lots of pounds in kit which you want ot control with the cheapest bit of tat you can find.

Would this get 5.1 sound from my tv and ps4 and switch automatically?
Highly doubtful. If the two optical carriers are both active at the same time the PLAYBAR won't know which to believe.

As noted, you get what you (don't) pay for. Buy yourself an optical (TOSLINK) switch and do the job properly.

A splitter isn't what you want at all. You need a switch. As the blurb says, it takes ONE audio source and splits the signal. You want something that picks one from several audio sources.

Look at this on eBay http://www.ebay.co.uk/itm/321740736081

Would this get 5.1 sound from my tv and ps4 and switch automatically? Presume when the ps4 is on if I tell it to go through optical no signal will come out of this tv ?

The advice given regarding the Optical Splitter is correct. Don't waste your money. You need a Switch. You may not want to spend this type of money but below is what I use:

Here's the link:
http://www.octavainc.com/HDMI%20switch%204x1%20ARC%20port_pro.html

One more point....
Getting DD 5.1 from the Playbar is dependent upon whether or not your TV will pass DD 5.1 over optical. No switch can make that happen. Check your owners manual or call the manufacturer tech support to be sure.

Good Luck....Cheers!

I ran with one of these until I changed my TV (to one that supports 5.1 passthrough);

https://www.amazon.co.uk/Portta-TOSLINK-Digital-Optical-Switcher/dp/B00KDZEWWO/ref=sr_1_3?ie=UTF8&qid=1476713242&sr=8-3&keywords=portta+toslink

This gave me 5.1 from Xbox One and Sky HD (if broadcasting in DD) and worked flawlessly.

I ran with one of these until I changed my TV (to one that supports 5.1 passthrough);

https://www.amazon.co.uk/Portta-TOSLINK-Digital-Optical-Switcher/dp/B00KDZEWWO/ref=sr_1_3?ie=UTF8&qid=1476713242&sr=8-3&keywords=portta+toslink

This gave me 5.1 from Xbox One and Sky HD (if broadcasting in DD) and worked flawlessly.

As a suggestion...just to be clear... you should add (so the OP understands that the Portta is not working any magic) that he/she would be required to run HDMI from the PS4 and the other source to HDMI inputs on the TV. Then select external speakers in the TV menu. Next run Optical from PS4 and other source to the Portta and then to the Playbar. In that setup the OP is by-passing any DD.5-1 optical short comings that may be imposed by the optical out of the TV.

I might also add that in order to get DD 5.1 for TV broadcasts the OP (if using a Cable box with HDMI and Optical out) would follow the same procedure; thus using the 3rd optical input of the Portta. Of course all the aforementioned requires that the TV have at least three (3) HDMI inputs to use the Portta effectively.

The Octava Switch (I mentioned) accommodates four (4) HDMI sources for picture and four (4) optical for Audio. The end result is One (1) HDMI to TV and One (1) Optical to Playbar from each source!

Cheers!

I ran with one of these until I changed my TV (to one that supports 5.1 passthrough);

https://www.amazon.co.uk/Portta-TOSLINK-Digital-Optical-Switcher/dp/B00KDZEWWO/ref=sr_1_3?ie=UTF8&qid=1476713242&sr=8-3&keywords=portta+toslink

This gave me 5.1 from Xbox One and Sky HD (if broadcasting in DD) and worked flawlessly.

As a suggestion...just to be clear... you should add (so the OP understands that the Portta is not working any magic) that he/she would be required to run HDMI from the PS4 and the other source to HDMI inputs on the TV. Then select external speakers in the TV menu. Next run Optical from PS4 and other source to the Portta and then to the Playbar. In that setup the OP is by-passing any DD.5-1 optical short comings that may be imposed by the optical out of the TV.

I might also add that in order to get DD 5.1 for TV broadcasts the OP (if using a Cable box with HDMI and Optical out) would follow the same procedure; thus using the 3rd optical input of the Portta. Of course all the aforementioned requires that the TV have at least three (3) HDMI inputs to use the Portta effectively.

The Octava Switch (I mentioned) accommodates four (4) HDMI sources for picture and four (4) optical for Audio. The end result is One (1) HDMI to TV and One (1) Optical to Playbar from each source!

Cheers!

You can sugar coat it with a dummies-guide all you like, but as a cheap option the Portta switch is a perfectly adequate cheap solution. Amount of HDMI connections was never mentioned. The use of the Octava box you mentioned is a possible solution, as is mine. I preferred individual HDMI sources connected direct to the TV as I could customise each connection independently (Game mode preset for Xbox in HDMI 2 etc). The Octava switch removes this option an means the source has the same picture settings no matter what your source is. Again, all subject to how many HDMI ports the OP has and how he wants to utilise them.

Didn't mean to suggest that Portta solution was inadequate. If it came across that way...my apologies. I participate in several community forums and have learned from experience that too much info is most often better than too little,

Cheers!

I ran with one of these until I changed my TV (to one that supports 5.1 passthrough);

https://www.amazon.co.uk/Portta-TOSLINK-Digital-Optical-Switcher/dp/B00KDZEWWO/ref=sr_1_3?ie=UTF8&qid=1476713242&sr=8-3&keywords=portta+toslink

This gave me 5.1 from Xbox One and Sky HD (if broadcasting in DD) and worked flawlessly.

As a suggestion...just to be clear... you should add (so the OP understands that the Portta is not working any magic) that he/she would be required to run HDMI from the PS4 and the other source to HDMI inputs on the TV. Then select external speakers in the TV menu. Next run Optical from PS4 and other source to the Portta and then to the Playbar. In that setup the OP is by-passing any DD.5-1 optical short comings that may be imposed by the optical out of the TV.

I might also add that in order to get DD 5.1 for TV broadcasts the OP (if using a Cable box with HDMI and Optical out) would follow the same procedure; thus using the 3rd optical input of the Portta. Of course all the aforementioned requires that the TV have at least three (3) HDMI inputs to use the Portta effectively.

The Octava Switch (I mentioned) accommodates four (4) HDMI sources for picture and four (4) optical for Audio. The end result is One (1) HDMI to TV and One (1) Optical to Playbar from each source!

Cheers!

Yeah but there is a few hundred difference in price! The system is already expensive enough I'm just after a cheap fix

The system is already expensive enough I'm just after a cheap fix
The optical switch suggested is a mere £14, whereas your £2 splitter won't work.