www.ultimatehandyman.co.uk

This is a cross post from the carpentry forum, I didn't know where was best.

single glazed window, thick frame. I chased the frame back and fitted dopuble glazed zunits to it.

Still masses of condensation problems.

Can (should?) I retro fit trickle vents, or just give up and get a new double glazed window from magnet?

yes mate, piece of cake, if youve got 20mm of room just drill a line of 8mm holes, get your vent and knife the nibs off, screw em on
got to ask though... how thick was the unit cos a little 12mm unit aint gonna do a great deal, the optimum is a 4/16/4 24mm unit..