I need to get a set of sockets for my family of Fabias. The 'spline' type that fits, for example, the seat frame to floor bolts. I have a 3/8 drive socket set to drive them and wondered if someone could tell me what they're called and a good place to buy. How are they 'sized' , what sizes to get in a set and generally what to look out for? I've seen reference to 'spline' but they often show 'Torx' which I've already got.

They are spline sockets, named M6, M8, M10 etc etc

http://www.ebay.co.uk/itm/FRANKLIN-6-Piece-12-pt-SPLINE-BIT-SOCKETS-1-2-Dr-TA562-/260875613678?pt=UK_Hand_Tools_Equipment&hash=item3cbd65f5ee

Laser, Halfords Pro, Franklin, Draper are all decent enough for the home mechanic. Or splash out on some Snap-On if you are feeling flash!

a set of snap on spline sockets is about £120 so that's prohibitively expensive for most people... i saw a set of spline sockets in halfords the other day for like £20 or something plus they sell individual ones too, as stated above they come in metric sizes, iirc the one you need for the seats in an M8 spline, which is pretty much the only one you ever use on a vw group car, the driveshaft bolts are M8 spline too, and the cylinder head bolts are an M10 spline