I kept hold of my GS2 luckily so now that I'm without my HTC One, I'm not stuck. Dial a phone have really been great though.
Called them to tell them that I wasn't happy with the volume rocker - they said no probs sir, I shall send you a postage paid bag to send it back and we will replace it with a brand new handset. Received the bag, put the handset in it and posted it off in the evening on the 24th. Just called them again to find out if everything is ok and the lady told me they received it into the warehouse this morning and she will despatch a new handset out to me right away which will probably be here by tomorrow morning
Grand total of time without my HO - 2 and a bit days. Not as good as swapping in shop but not bad at all.