thanks, Sean, Matt and Steph, I'm sure there will be further questions.
In the meantime the smaller (2' 8") wheels arrived from Roxey, insulated one side only which should dispense with wiper pickup. If I've understood the drawings correctly the floor height should be 1144 or 26.3mm in O gauge, and that's what it is, with the new wheels. And they seem to look right with the bogie frames as well.
Off up to Sheffield in an hour's time, so hopefully I'll see the real thing. I need to check if Northern Rail has any 150/2's with 2+ 2 seating, as that is what I ordered with the kit.