thermostat housing (should be easy)

[andym]

OK, hopefully it should have an easy answer.

The part in question:
https://img.photobucket.com/albums/v320/andy36586/Thermostat-housing_zpse8736c3f.jpg

The last time I fitted this properly the engine was off the bike, but now every time I fit it the joint between the housing and pipe (conveniently pointed out) leaks, I've even tried using silicone seal around the rubber seal, but that doesn't work either. The only reason I can think of is because of the space involved I'm having to tilt the whole thing and build it up in the frame which must be knocking the rubber washer out of line or something, as I have the spare parts and I've built it up outside the frame and it all sits together more securely. There just isn't enough room under the frame to fit it while built up (unless I remove the carb and airbox which took me days of blood, sweat, tears and swearing to fit last time, and even at that it was fitted with brute force and ignorance, which resulted in a couple of little splits in the rubber).

So can anyone offer any advice on how this is supposed to be fitted with the engine in place? I've read the manual and that is no help. To give an idea of the space involved, the top of the housing sits higher than the outside of the frame, and is stopped at the bottom by the carbs... when it is built up in place there isn't even enough space to get a spanner over 2 of the nuts.