New fishkeeper, advice sought

Hi Folks,

I've recently been feverishly browsing the net looking for info on setting
up a freshwater tropical tank, but I can't find the answer to this
particular conundrum on any website and can't rely on the advice of my local
fish shop.
The fish shop in question sold me a betta, pleco and small (approx 10 litre)
tank three weeks ago. I went in believing that a betta was a delicate
tropical species and came out having been told it was a hardy 'all weather'
species which could breathe air and was used to a natural habitat of mud
ponds- dark and murky. Lies, I now know, but not in time to save my poor
betta who succombed to a fungal infection the day before we were due to move
him to a new home - a heated, lit and filtered 50 litre tank.
The pleco survives and is in the tank, since my newly purchased ammonia and
nitrate tests (and the dead betta) showed that the old tank wasn't safe by a
long shot.
So now the pleco is in the tank with two plants, mod cons, shelter and a
piece of cucumber which he is ignoring. I want to cycle the tank fully
before introducing any new fish and I'm aware that plecos aren't an ideal
species for this (ideally I'd be doing a fishless cycle but it's a bit late
for that). The question is, is a 2" pleco going to be ok in an uncycled 11
gallon tank while the bacteria get their act together?
The tank has had AquaSafe chlorine treatment, "Stress Zyme" bacterial
supplement (though I've recently read that such supplements are pretty
useless) and a dose of anti-fungus, in case the pleco is at risk from the
infection that killed the betta. The funcicide is phenoxyethanol based and
claims to be "harmless to filters and all species of fish and plants".
A small amount of water from the old tank will have made its way into the
new tank while transferring the pleco, though I think the problem was water
quality rather than any virus or unusual pathogen. I still have the old
tank, in case I find out that I can speed things up by recycling the gravel
(after a good rinse) - perhaps I could test it out with a dose of ammonia in
some fresh (dechlorinated) water?

My current plan is to keep testing, replacing a portion of the water
depending on results, until the tank stabilises.

Thanks in advance for any advice you can offer,

David

PS. Can anyone recommend a good fish shop in London?