It seems the problem was down to a dodgey batch of neons from my tried and tested LFS.
Thursday I bought 6 neons which appeared healthy (always check for visible signs of disease or damage - along with the tankmates at the LFS). Introduced them to the tank, all was fine until Saturday when I noticed two of the new neons had a white patch, looked almost as if their skin was faded (not slimey neither was it whitespot) - one had this white patch on the tail and another on 1/3 of the top of the main body. Instantly I did a water test, partial water change and treated with a course of interpet anti-bacterial.
Water tests came back fine (nitrate fine, PH perfect, nitrite a bit high but OK, amonia perfect). Over the next few days I have lost

7 neons, 1 clown loach, today I lost my last big gourami and obviously poor flash on Sunday.
I had no fatalities yesterday (other than 1 neon that died over night) and thought the problem was under control but my last big gourami died when I got home tonight.
None of the fish had any of these white patches (the loach that died seemed to have similar white marks although hard to tell as was faded when I found it) - all the other fish look perfectly healthy and are acting normally - my pearl gourami doesn't look too active (although that's nothing new, he's a lazy tike) but all others are acting normal.