Stu, my problem was the rims. EX721's are built for downhill mtb'ing, and can take up to 3" tyres!!!
The 1.25's are barely wider than the actual rim, so you can imagine what Robin's reaction was. I'm guessing the risk is of the tyre rolling off the rim under extreme lateral pressure?
The other issue is pressure - at 3" the maximum pressure the rims will manage is some 40psi, so I was worried about running narrow tyres at high pressures, having read about rims going pop!