Make sure you remove all the lose plaster then give it a wash of one part PVA to 5 parts water. Let that dry overnight then give it a coat of four parts PVA to one part water and, whilst that is still tacky, skim over with multi finish plaster. Make sure you get the wall absolutely flat, large tiles are completely unforgiving of any out of true.

Alternatively, if you are going to tile the whole wall and your plastering skills are not perfect, you could glue a tile backing board to the wall.

I would use some Blue Grit instead of PVA.

Given the hassle in removing the rad, replastering, redecorating...I'd always play for safety and use Blue Grit.