Actually it wasn't getting the new tyres on that was the issue, it was getting the old Pirelli Phantom Sportscomps off. They were incredibly stiff & didn't want to come off the beads. After evetually getting the beads released, the first side came off OK to get the tube out, then I really struggled to get the second side off - for the first time I bought a tub of proper tyre lube paste, but the edge of the tyre just seemed to bite onto the rim, not slide over it. Both wheels were the same.
The new Continentals slid over the rim without using the levers on the first bead & didn't put up a fight after fitting the tube to get the second bead on. I wasn't happy with the position of the valve stem on the front & deflated it to re-position the tyre - I could push both sides (freshly lubed, admittedly) off the beads with my thumbs. The Pirellis needed a pair of G clamps "walking" more than half way around the tyre to get them to come off.
Still got the pair of Avons to fit to the Classic 500 and will be taking wheels & new tyres to the local independant tyre guy - £20 a wheel to swap out tyres I have supplied is a bargain compared to todays situation.