I have Campagnolo rims on my racer which seem to be a slightly larger diameter (68cm approx) than most 700c rims which is causing havoc for me when doing tyre changes. Do you know of a tyre that is slightly larger than the norm that would fit these rims better enabling easier tyre removal?
No, I don't know if "easier to fit" tyres. But I'd say that steel bead can be considerably easier than kevlar bead, especially when new. Mostly I'd urge you to check your rim tapes. Some are good (Velox cloth tape), some are crummy (any sort of rubber tape for example). A common mistake is to add a good tape over a crummy one, making tyre fitting quite difficult. It's not so much the outher diameter of the rim which determines how easy or difficult a tyre is to fit, as the amount of movement the tyre is allowed within the rim. A thick rim tape makes a tyre harder to fit, 2 rim tapes can make it almost impossible. At a pinch, you could use the Ritchey rim tape which is thinner than the Velox, but I don't think as reliable; it's a solution we've not had to resort to yet. Also, when removing a tyre or in the final stage of fitting, squeeze ALL the aire you can out of the tyre and close the valve so it doesn't fill again. Wrap your chest and arms around the tyre and have it pushing on the floor. A little bit of air makes a big difference. Good luck!