Well I've solved it.
We live in a terraced house and the house next door is a holiday home. They were here last weekend. Normally when they go home he turns off his router, but I guess he accidently left it on this time. It's a BT Smart Hub 2 (same issue). All of this coincided with my transfer to PlusNet so I put two and two together and made six. I unplugged everything this morning including all the Tenda Mesh nodes and 'hidden network' was still there. It was never my router at all.
Just goes to show, these things often turn out to be the thing you least expect.
Oh well perhaps I'll stick with the PlusNet router, pretty coloured lights and all!
