I have posted this question as advice for others. I used to have my guinea pigs' claws trimmed at the vets some years ago, but I have since found an easier way, and my youngest 'pig who is at least four years old has never needed his claws trimmed.
My solution to this problem was to put a sandstone tile in the cage. every time the guinea pigs go to their food bowl they shorten their nails just a little, here is the layout:
and here are their feet, as you can see they are quite healthy:
The guinea pigs' pads prevent the claws from getting too short, and the constant wearing down stops them getting too long.
No trimming claws.
Less stress for your pets.
Stone is heavy to move when cleaning the cage.
Animal claws are naturally sharp.
Because you don't need to groom their claws, 'pigs can become less social.