When Locrep filled it, he stuck some lovely neon-green dye in (I swear I had socks the same colour back in the day...

). After a day of running it, I saw a leak. Lovely bright green ooze on one of the pipe joins between the compressor and fixed pipes attached to the body.
Call to Mr-T, "yup, you'll have to replace the pipes. Happens often" For the two, it came in at about £350! Not even Ian could ease that one (I had not yet asked...)
Some peering at the EPC and see there is an o-ring between the two pipes. £2.50 or there abouts for the o-ring.
A trip back to Locrep to remove all the refrigerant, 60 seconds to swap o-ring (really, it was that quick) put pipe back together, refill with good stuff and I'm away.
So far, a week later and lots of aircon and not a sign of a leak.
Thanks again for your help Locrep!