Yesterday I was lucky enough to be able to borrow a new new set of Campagnolo Shamal two-way fit wheels from a mate for 6 months. I went to fit a new set of Conti GP4000s tyres I had and after a fair bit of swearing and some blisters I gave up. It was 11pm so I can inside and started researching the problem.
It appears that due to the coldness of the shed, the tyres wouldn't stretch as they normally would. So I took the wheels and tyres to work, left them in my boot in the warmth all day and in the afternoon, easily finished the fit.
Moral to the story - don't fit campy wheels in cold conditions!
Hopefully this saves a few people some time of the future