As promised, an update on the drainage issue (unable to reply on the original thread as its been archived).
We got a contact who knew a self-employed guy who had a camera that'd do an inspection. This revealed a concentrated lump (I suppose, a fatberg, as they seem a common talking point). Not sure how it had accumulated - nor over what amount of time. He tried jetting it - to no avail. Then several doses of highly concentrated drain cleaner (lethal stuff, which no doubt body-snatchers use). After several sessions with that the blockage finally gave up and admitted defeat. Big sense of relief as at one stage I had a feeling that it'd require lifting stone floors etc (complete with under-floor water pipes etc). The stuff that finally did the business is available commercially - if anyone wants the details I can supply them.
Cheers
