Decent app for what?

Pretty much any modern NAS that supports SMB will do fine for Sonos.

Visit Amazon or other site and find one with good reviews and in your price range.

Thanks Stanley

Is one better than another? Price isn’t that important (ish) I’ll be using a 1tb SSD

Check out the link below. It may help with your decision.

Network Attached Storage (NAS) Buying Tips | Sonos Community

If you are planning on using a 1 tb SSD that you already have or plan to buy, you are not looking for a normal  NAS, you are looking for a diskless NAS chassis. If you don't know exactly what you are doing you will be presented with a "learning opportunity" also known as a real pain getting it set up. You would be far better off buying a normal NAS that includes the size drive(s) you want.

You may also want to consider getting a two or more drive NAS so that it can provide some data protection that a single drive device does not.

Good point. 
in my naivety I thought that an SSD would be better with accessing music files. I’m happy to stick with a standard 2 bay mirror ÑAS. I have another old one set up for work

Idon't understand that at all, why are you thinking SSD drives in a NAS would be a bad thing for Sonos?

My music library lives on a SSD drive on a computer dedicated to serving as a NAS and it functions perfectly. I particularly love the low power, lack of heat and noise and small size you can get in an SSD NAS device.

Unless you have other NAS needs that require a spinning rust drive they are a poor choice for Sonos use. 

Sorry you’re right … My mistakje.

I’ve decided on a Synology DS244+ NAS with 2x 1tb SSD drives running as a mirror image. 

It will be interesting once I’ve set it up as I live 4 weeks UK and 2 weeks Spain… Will I be able to access the Synology NAS in Spain or do I duplicate the system. I know in theory I will be able to stream from my UK NAS but I may get buffering. I’m running the same Sonos set up in both houses.

Sonos cannot remotely access an NAS drive.

You can't directly access anything not on your local network but…

If you add an SMB gateway to your second location Sonos can access it. You would need to connect the SMB gateway machine to your home NAS using a protocol (like NFS) that will connect over the Internet. 

Setting up the gateway machine isn't rocket science but does require basic networking, NFS and SMB skills. I've done this gateway locally but not remotely. This is old and may need adjusted for new settings.

https://stan-miller.livejournal.com/357.html

Ha ha. Let’s get location 1 set up first

Sonos is designed to have everything on a single subnet. As long as the controller, speakers, and NAS are on one subnet, you should be fine. The networking becomes more complex, as suggested, when you add other subnets/locations, which is technically not part of Sonos’ purview. 

Thank you all. 
ill buy the NAS (through work so it is free) and let you know how I get on