Thanks for all your replies and help with diagnosing the problem with
my tank water.

Here's where I've got to now.. (with a few questions, if anyone can
lend a hand please..)
I removed all plants and discarded all those that were not healthy or
had algae (i.e. most of them).
Pulled out the undergravel filters/powerheads.
Hoovered all the gravel over the last 4 days causing a 25% water
change/day (a bit drastic I know) and it looks pristine.
One thing that crossed my mind - I've got on average about 2" gravel -
is that about the right amount?

Replanted, cleaned glass and flouro tubes and replaced mylar (posh
baco foil) reflector on lid.

I put all the driftwood & bark into a fairly strong bleach solution
for a day then have left it on the shed roof to wash in our great
British weather - can I re-use the bark/driftwood ?? It's been well
washed over the last dew days!

Today I installed an Eheim 2329 wet/dry (bio) filter (what fun that
was! and why do they supply such short hoses? I had to spend £14 to
buy new ones), the biggest surprise was how much the water level goes
up and down as it 'breathes' - I'm hoping this, in conjunction with
the Eheim 2217 and removal of the UGF's will keep my water sweet.

I've ordered a pair of 150W metal halide lights that'll help with the
plant growth. They're 2nd-hand and I'll have to replace the bulbs,
does anyone know what I should be looking for in terms of K rating? -
for freshwater tropical, so the MH is mainly there for the plant
growth.
Should I also continue to run the 3 x 36W Triton's in conjunction with
the MH lamps for about 10hrs/day?

At the time of writing the fish all seem very happy - such lovely
smiles - and with all the manky plants gone, there's so much space!

Thanks in advance for your continued help
Pete