Ok so good news, they did the usual ultrasound to exactly place where the cyst was, if it was pushing on any nerves, arteries etc. It was near the point where two muscles join which explains the difficulty I was having with my grip. My attempts to drain it myself were on target but the stuff in the cyst had hardened really quickly for some reason. They delay dealing with it sometimes depending upon various factors but this was too near an artery so they called the specialist who took a look and didn't want to leave it incase it grew and cut off the circulation. I thought they would end up cutting the muscle open but before that they tried injecting something or other into it which helped to loosen things up and then it could all be drained off.
So yeah, happy to say I'm fine, no surgery needed, my grip is back to normal and generally feeling ok. The arm is very sore because they had to palpate the area after injecting stuff, then push hard when draining it etc. But hey it could have been a lot worse.
For all the criticism our NHS gets I think they're great.