Upon further searching I have found that Tamiya do a gloss clear green pot of acrylic paint (called MiniX25 or something), I've found one on Ebay for £1.60 + £2 shipping so I've ordered that. Not sure of the shade of green, looks a bit dark but I will give it a go on some spare clear sprue and if it's any good I'll perhaps see if I can't use my dremel to gently remove the existing paint on the cabin roof windows I've already painted.
Tamiya also appear to do a translucent green spray for use on polycarbonate RC body shells, which as far as I can work would be similar to that Testors one, but as it's about £5+postage I'm not going to try that out as it's a bit expensive considering what I'll use it for.
Thanks for the tip though Gino, much appreciated, I thought it had to be something like a ready mixed spray or tin of clear green, that most people use to achieve that green window effect.
Last time I read an instruction booklet to find out what paints I need though. Lol.