I finally got it home this afternoon. It appears to be called an Agressor XCR. The bits are low-end, SR Suntour chainset like I bought new in 2008, Suntour forks that pogo (not so nicely) but I have a set of replacements in the garage. Tektro discs (I haven't yet checked the calipers) which aren't worn at all. I haven't checked the transmission either. 8 speed. And I think the bars are well wide!
Nothing appears seized so far!
ETA: It's an Agressor XC3 which was an Avalanche rebadged for marketing by Halfords. The 3 was the entry level, the 1 seems to have been reasonably well-considered. It weighs a ton, so much heavier than Phil's Rockhopper Geral which is a hardtail that must be a decade older. That's progress! The GT appears to date to about 2009 (there's what I take to be a production code starting M09, as well as the serial number and the components are about that vintage).
It needs a lot of penetrating oil, the cables are rusting, a cable adjuster on the rear caliper is well seized, the front mech isn't moving freely enough to get the granny ring and I'm sure the chain will need replacing (probably cassette, maybe chainset as well). The good points - the frame has rack and mudguard eyes.
It may be worth the €0000.00 that I paid for it but perhaps not a lot more!
That's all