In reply to duchessofmalfi:
Thanks to Naomi's post, I did some diggigng and it very much seems to be Olecranon Burisitis - as you say.
I think what has happened is that about two weeks ago I knocked it on a boulder - not while climbing. It was such a minor thing at the time that I thought nothing of if and can't really remember what I did.
Since then it's hurt a little but not at all while climbing.
Since it went 'pop' a couple of nights ago, it has been painful enough that I lost a couple of hours sleep and woke up with it looking quite swollen.
Digging around the net, there are a few instances where people have whacked their elbow and it has affected their climbing rather than climbing being the cause.
Mine's definately a minor case compared to some of the stuff on YouTube! I think I'll ease back to climbing but may dig out my mountain bike elbow pads