I do have a bit of good, fun news .
Got a text from my mate today who had gone to my house to work on his car (in my garage). He has keys & went in to use the loo, and was hit by the strange smell in the conservatory - I'd noticed it the other day but rarely go in there. Coming back through, he spotted a mouse!
He rescued the sacks of dry cat food that live out there & after some poking around found droppings etc. So when I got in from work I let the cats in there - and Girlcat has caught her first mouse! She paraded round me with the very dead corpse dangling from her mouth then vanished, so presumably there's some disemboweled remains somewhere under some furniture. 
I've shut them out for now, and the conservatory is off the kitchen which is out of bounds during the night & when I'm out, but I'll give her another go tomorrow evening. I THINK Boycat helped her too - he was on guard at one end looking impressive & sniffing loudly 