In reply to PPP:
A 17mm-19mm double-ended ring spanner is the best option, nearly every bolt you will find in the UK is 10mm (which need a a 17mm for the nuts), some bolts are 12mm (19mm nuts). Any other spanner will be for maillons which have very odd sizes, for those a small adjustable is useful.
The advantage of a ring spanner is that they are really easy to clip to the harness, the disadvantage is that some cheap, discount store type spanners are too chunky to easily fit on the nut with some hangers, good quality spanners are slimmer.
I applaud your desire to take a spanner, they should be part of every sport climber's kit, in the same way that a nut key is part of a trad climber's.