Sorry, I think this thread already answered my question:
//www.diynot.com/forums/viewtopic.php?t=50490
...always the way, you look everywhere, then when you stop looking, you find it!
I am building an extension, with full planning permission etc, can I fit my own upvc windows or do the new regs cover that too?
The two I need to replace on the existing house will almost certainly need fitting by a registered company as you have suggested, so probably makes sense to get them...