Went straight down to the bottom at 35 m and went to investigate one of the hydro-boxes that used to pump out water when Stoney was a working quarry. Vis was pretty bad - we needed torches - but it was quite exciting scrabbling around under a hydro-box in the dark. Suddenly John signalled to leave and ascend. We hurried out and I was curious why he wanted to get out so suddenly. Started to ascend but found I wasn't going anywhere (stayed around 28 m). Found a line and hauled ourselves up. At the surface I discovered my stab jacket was belching out air at the back due to a jammed dump valve (hence my lack of buoyancy). John's regulator had free-flowed in the hydro-box (hence our rapid and sudden departure).
Looking back on it this dive could have gone horribly wrong - we were very lucky even though nothing bad seemed to be happening at the time. Lessons learned: pay really close attention to your buddy. If it's dark in poor vis and you feel happy it doesn't follow that your buddy is happy too. Keep a close eye out to make sure everything is all right. I had no idea John's regulator was free-flowing, partly because it was so dark and I couldn't see him, but also because I was having such a ball it didn't even occur to me that there could be anything wrong.
Also, don't rule out equipment failure. I was squirting bits of air into my BC and dumping at appropriate time intervals for a slow and steady ascent. But the fact that I was actually ascending even though I was finning up and squirting air into my BC should have alerted me to the fact that there was something wrong with my stab jacket. Because of a slight delay between injecting air and it belching out of the back of the jammed-open valve at the fact I could have made it up unassisted by holding my finger on the direct feed button continuously, and next time I'm not making any headway going up I'll know to do this. I should have paid closer attention to my instruments. I was so conditioned to seeing numbers go down when finning upwards and dumping a little air, that it took me too long to realise that in this case I wasn't actually going anywhere.
A combination of good luck and the fact that I didn't realise anything was going wrong (which meant I didn't panic) meant that this dive ended without incident, but looking back on it, it could have all gone horribly wrong: if John hadn't kept his head when his regulator free-flowed or if we hadn't found a line to pull ourselves up or if anything else had gone wrong giving me a need to ascend rapidly we could have been in really big trouble. While we were on the surface a rescue of another diver was taking place. They had to put him in the pot at Stoney Cove and an ambulance arrived. This brought home how easy it is to get hurt in this spot if things go wrong and you fail to take the right course of action. A very sobering dive.