I am about to add a spanner into my kit while sport climbing outdoors. I have been to only one sport climbing venue so far, but I think it's worth taking a spanner for loose hangers/bolts. It's better having one rather than coming back home without finishing a route, that's for sure!
However, the more I search on Google, the more confused I get. I have found that people take 14mm, 17mm, 19mm spanners... DMM Nutbuster ( http://dmmclimbing.com/products/nutbuster/ )seems to use 17mm spanner. It seems like a good option as I will also need a nut key in future anyway. I'm just not sure whether this is a right size or not. Anyone has some experience with that?
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.
Only one at a time though, assuming all nuts on route are the same as the first one which you could check from the ground and just take the one (in the UK....less so at Buoux but the thread IS about UK sport )
Open spanners place too much pressure on only 2 flats so you should only consider a socket set for this task and an accompanying torque wrench to avoid over tightening. Don't buy no cheap stuff though, laser or black spur brands are rubbish, only get Snap-On or teng. Its your life after all so you need to pay for the best