It depends how much you're going to be using them and what other tools you have, but for occasional use I wouldn't be looking at spending £30 on a set. If you put "spline bit set" into Ebay you'll find plenty at around the £12 mark. I have a set like that (which were only £8 a set five years ago) and they have held up well.
They're triple-square drive bits, commonly described as spline drive. They are usually very tight too & if you use the incorrect bit it knackers the head very easily. Spline drive & torx drive are different.