£130 is cheap for a tray. Having said that trays costing 3 times the amount can vary in size. Just get on with fitting it and adapt the plaster board so it fits
Easiest way if it's a one off is for the customer to pay the labourer direct. Otherwise you'll need to register them up for CIS and even if you don't use them you'll have to do a monthly return.
No it wasn't.
He got advice from people who know what they are doing such as the need for a pump and the temp being too hot without a blending valve which he instantly dismisses because he can use google
Stelrad do a single panel without convector, I think it's their Elite range. It's not roll top though.
Personally I would replumb and get a rad with a convector
Edit - reread, I see you've found the Stelrad one. Agree looks horrible