Thank you to those who took their time to try to assist. Thank you Ken G for your pointers.

I thought I would post an update that would be helpful for anyone having the same issues & share the  fix.

Problem - The wireless sub mini would not connect or stay connected to my wireless Beam 2.

This is how it was resolved….

I connected another Beam I have in the house (different room on a different floor) to the internet with a data cable into one of the Rukus satellite access points routers, that all link back to my main Drayfus router.

This immediately changed my Sonos system status from WM1 to WM0 & created a SonosNet Channel now shown in my System / Network. This is the key to it working!

That’s it. Added the sub mini to the system, connected to the Beam 2 in my cinema room & just like magic, it all worked. And I have to say, despite my frustrations, I’m pleased I persevered as it sounds amazing. Set up True Play & it really does sound like a cinema experience. Awesome.

Sonos support have been very helpful & have responded to every email.

If you are buying Sonos equipment, I would recommend Smart Home Sounds where I bought the new sub mini. Amelia from customer support has been amazing, I highly recommend them if you haven't yet bought & they also do a 6 year warranty included in the price. 

I hope this helps anyone having the same issues.