I'm in agreement with Fikealox that the L20H really doesn't have enough clean headroom for gigging.
I'm also in agreement with HTH that you certainly could be looking at an older 70's amp. Something like a Selmer T & B would be great (built like a tank, cheap, perfect !). Pick up a Selmer for £200, get it recapped and checked over by a tech (£100-£150 I'd guess) and you would have an awesome and original amp, which will only appreciate in value.
You could certainly pick up a Marshall JMP 2204 for your budget, although I would also advise you get it recapped, which might push it above your budget.
There's also a superb 2204-modded Wilsic Sound amp (yes, it's mine !) on ebay at the moment (hand-wired, re-capped, fully sorted !), which will last a lifetime and should come in at well within your budget -
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MESELX:IT&item=180318465973