Ok, so I got brave and brought it up to D standard to have a proper look (wearing safety glasses, a hard hat and a high-vis jacket on of course - I'm not
that brave).
The belly is pronounced, but not that bad a D.
What is bad is the gap all down the back of the bridge (only photographed the corner so you could see 'under' the end but it's all the way across.
At this stage I'm thinking this guitar really is end of life. Its a shame, but for what it's worth (<£100), and with no emotional attachment (i only met it yesterday), I can't justify the time for me to fix this or the cost for someone else to
Only other thing I can think to do it string it with nylons -- at least they're unlikey to pull the bridge off while I'm playing it. I've never owned a nylon string guitar (or really wanted to for that matter). hmm...