Well I thought I'd share my experience with everyone - hopefully it'll help some other noob.
After reading this on another site
http://www.brewuk.co.uk/forum/topic/twe ... post-30304 I decided to check the cap.
Yes I had overtightened the cap, the seal/washer was deformed and not seating properly. I followed the advise about reshaping the seal/washer with boiling water and cooling it back down. See link within my link. I now needed to re-prime the barrel to get some co2 back in.
I prepared my brewing sugar (70g), not caster sugar, to re-prime the barrel. Using a funnel I dumped the sugar on top of the beer, and thats where the fun started. It was like dropping a mint into a bottle of coke
It erupted! About a pint and a half of beer gushed out all over the airing cupboard floor! And to make it more fun, I was doing this at 12.45 in the morning.
After a minute or so the froth died back down, and it took me about another 20 mins of cleaning up, luckily the cupboard isn't carpeted. I reckon the brewing sugar, being much more like caster sugar (very fine) pretty much dissolved on contact and caused the eruption/reaction, it might not have done this had I not just dumped it all in in one go, I don't know.
Anyway my dilemma now was how much sugar stayed in the barrel, not much I don't think, it just came straight back out. So this time I added caster sugar (50g) SLOWLY. My theory that caster sugar will dissolve slower. The beer didn't react violently this time :clap: I gave it a gentle stir then finally screwed the cap back on.
Cap screwed on carefully this time. I applied a touch of cooking oil to the seal, screwed on the cap until you feel the seal bed against the barrel neck then 1/4 turn. You can actually see the seal through the cap, to make sure it's still in place. Went to bed at 1.45am on a school night, house smelling of beer :nono:
So I hope this helps someone with no pressure. Check the cap and easy when adding re-priming sugar!