Well, it seems it may be back to the drawing board.
After being persuaded that it was best to leave the boiler where it was, I got the tape measure out today, and there is only 530mm between the window and the drain pipe.
I require 300 (distance from window) + 100 (flue diameter) + 150 (distance from pipe) = 550mm, so am coming up 20mm short.
This could possibly be overcome with tonybhoy's suggestion of having the flue terminate proud of the drain pipe, but I'd have to check this was acceptable (both by the boiler manufacturer, and in terms of the listed building consent).
If anyone is willing to give non-specific general information about the pros/cons and possibilities of flue runs and flues up chimneys or wall cavities, it would be appreciated. I'll also investigate the possibility of moving it to the bathroom, but I'm not sure there is space between the wall and window to mount it.
Finally, accepting that I'm going to have to cave and get someone in to take a look, when I pick up the phone to talk to them, what exactly do I ask? "I'm looking at getting a new boiler, and probably relocating it, and was hoping your could come and have a look and offer some advice" - or would I at least need to tempt them by saying I'd want a quote?