Finally! After over a year (when we visited MiniSport, who told me the engine is down on BHP and compression, and the first thing to check was valve timing), the camshaft timing was checked. It looks like there was indeed a big problem in this area, namely a missing tensioner for the timing chain, which already caused a lot of wear on the chain. A new chain went in, along with the tensioner. The engine seems livelier, now, which is good news.
The other good news is that the clutch bearing, which started to sing
whenever the clutch was depressed, has been replaced. Turns out there
was a lot of wear there too; the clutch felt positively light now, and
my mechanic assured me the orange diaphragm is still in. The old
bearing felt thoroughly knackered, and the word was that my decision to
not go on holiday with the white Cooper and take Demelza's Mini instead
was a good idea, because it would definately given up.
There's some bad news too, though. The compression hasn't changed a bit
since the last time, and that shouldn't be a surprise according to my
mechanic; if valve timing would be as much off that it would start
affecting compression, the engine wouldn't even have run.
It feels like the car has gained a few BHP, but due to the compression
that is still off, I doubt it is where it needs to be, which is in the
80+ BHP range. It might have entered the 70+ range now though.