Rob, I don't know why rock pegs keep getting suggested as a general solution - I've noticed it a lot, and it's daft unless the ground is so hard that nothing else will go in, or someone wants to nail their tent on their decking.
The 'V' or 'angle' pegs have a v shaped cross section like this:
It gives the pegs a greater contact area with the ground and provides a lot of additional strength. They're very light too - 6 inch alloy ones come in about 10g each.
Lor, there are all different sizes available - usually 6 inch and 9 inch. also, there are heavier duty Atlas pegs that have an additional spline down them that makes then really tough.