1990 Vauxhall Cavalier condensation

Assume there are no damp carpets ? Was going to suggest checking cabin / pollen filter but dont think that year of car had one .

You're right - no filter. There is a finger of suspicion in the direction of the carpets. Not visibly wet on top but perhaps moisture getting in underneath in spite of the underseal. [Would this cause condensation??] I will need to find someone in or near Newcastle on Tyne / Durham who knows what to look for in such cars - and if necessary is competent to do something about it!

Check any bulkhead drains , usually in back corners , are not partially blocked .If blocked , water can work it's way into car under carpets behind dash and go unnoticed .If you have a recirculating setting on the heater , turn it off .

Is that a DIY job and/or would I need a ramp? Could I get a diagram from somewhere showing what to look for?

I think my heater is a simple one without any recirculate mode as on more sophisticated vehicles.

Simple job to do .I tried to get pictures but hard to find for that model now .This link for later model might help you ,

https://www.youtube.com/watch?v=XxOxrSFIemg

And good description here of location of drain ,

" Check the flat rubber drain hose under the bonnet, which drains the scuttle
where the blower intakes are, wipers, etc. It's central on the bulkhead and
drains over the steering rack. I had a very bad "leak" which turned out to
be this hose clogged up with tree crap.

Coincidentally, I noticed a wet front passenger side carpet yesterday in
mine ('95 cav.) I found the flat hose you refer to above, but it was
****ing down here yesterday so didn't spend much time looking.

The hose seemed to have glued itself together at the end so that it
formed more of a colostomy bag than a drain, IYSWIM. Squeezing it to
force the end open resulted in a torrent of water and muck. "