I don't know the size for a Polo but it's probably almost as cheap to buy a set like this:
http://www.halfords.com/workshop-tools/tools/socket-sets/laser-7-piece-hex-bit-set
You're a bit vague about the type of bolt - all the ones I've come across on brakes have a hexagonal hole in a bolt head (usually called Allen Bolts), as opposed to a hexagonal head on the bolt, hence the above recommendation.
Years of working on cars taught me that it's cheaper in the long run to buy sets, rather than individual tools; you generally get a box to keep them together in and you always have the size you want. Incidentally, the cars I changed the brake pads on all had 7mm allen bolts which never seem to appear in sets of keys, so check carefully before you buy.
If you go to a shop like Halfords you may find a "VW Polo Brake Tool", which will cost you about as much as a set.