Blighty - get yourself some keg lube. It's worth the money. At the rate I've been using the stuff, I should get 10 years out of the little container I got.
When I first got my kegs, it was obvious they were fresh out of the soda industry. The tops had a ton of little dings in them from knocking them loose and I could immediately see why. It took two solid whacks with a hammer onto a chunk of wood to free them.
Since I put the lube on the new lid rings, I can open the lids by applying pressure with my thumbs. Plus, I haven't had any leaks. It barely takes any lube - I just rub a little on my fingers and then roll/drag the o-ring through until I can see it's completely covered. Well worth the cost.