The architect got permission from the owner for me to remove the kit. She said that he might be interested in "buying" some of it back once restored on account of the 'provenance'. The house and equipment was previously owned by someone who was quite well-known in the 80's.
That suggests to me that he owner DOES care - not because of it's value but on account of its provenance.
Now, if the owner may want to "buy it back" they're not going to give it away for free and then buy it back at cost are they?
No. They will "buy it back" on the understanding that it some work has been done on it and they'll be paying for just that because they own the rest.
Perceived value doesn't count any more because eBay is here - just put it on and it will find a price. Done like that, you'd easily get over £1500. Sold in a way to make maximum profit, more like £2500. The owner will care and want their stuff back once they find out, even if they don't realise the value right now. They'll want it back anyway when they start thinking about the provenance of the house itself..
So, unless you have clear, direct instruction to take ownership of it, without ANY doubt and either straight from their mouth or in writing, then you have possible accusations of theft on your hands. Unless you can proove they gave permission you're stuffed..