The good news is - Autoplas still exists, and although they no longer make louvres they will make up universal kits for your model of car with the fitting clips. I e-mailed Autoplas last night, recieved a reply today and had a long chat on the phone to a really helpful gal who said they'd make some reproduction ones for me tomorrow

I've ordered both a Mk1/Mk2 and a Mk3 set, just to see what the difference is. When I get them, I'll post up some pics so we can compare them with the originals.
[20.10.2011 EDIT: As far as I can tell, Autoplas never made MkIII hatch louvres - so no luck in ready-made fitting kits for them!]
Makes a change from the usual "no longer available, sir" reply, when looking for V300 parts

http://www.auto-plas.co.uk

Cheers
Tom