Well, hopefully we've finally got to the bottom of it (touches wooden desktop!).
The garage have accidentally (?) left a piece of paper on the seat which appears to contain the fault codes they read, and their interpretations of the possible faults:
P0300 - random multiple cylinder(s) misfire detected - HT leads, coils, low compression or wiring.
P0304 - cylinder 4 misfire - engine mechanical fault, wiring, ignition/fuel system, injector, ECT/MAF sensor, ECM.
P0351 - Ignition coil A malfunction.
Bear in mind that the coils, HT leads, plugs and sensor have already been changed.
However, they reported that the engine was very wet (with water), and on investigating, found that all the drain holes for the air chamber between the bulkhead and the windscreen (fresh air intake for the cabin) were blocked!
What appears to have happened is that the chamber has filled up with water when it has rained, and eventually its overflowed, showering all the electrics under the bonnet with water!
This makes perfect sense, as its only since the weather has changed that its started acting up!
He's cleaned all the drain holes out, thoroughly dried the area, then sprayed WD-40 over all the nearby electrical cables.
The car is now running like a sewing machine again, with no warning light. Hopefully that's it fixed. But if it recurs, I will report back!
Hopefully my experience may help someone else with the same issue. Its not an obvious cause for the effect observed, and to me, indicates the problem with relying too much on computer diagnostics!
Many thanks to all for your helpful suggestions.
--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!"