Problem sorted - I think!
The 'noisy gearbox' problem first manifested itself whilst driving home in the rain on Sunday. I drove the car today and with the exception of the noisy tappets (this weekends job), the noisy timing chain, and the slight exhaust blow from the manifold flange, it was pretty quiet. Then the rain started. So... windows closed. Quarter-lights closed. Wipers on. Hey presto! The noise was back. Seems to be a combination of wind noise (not mine personally, but ill-fitting quarter-light seals, ill-fitting passenger door weather-seal and ill-fitting gearbox filler rubber plug), a very noisy wiper rack, and the general sound of surface water against a thinly-insulated floor. Combine these elements with the existing tappet, timing chain and exhaust noise, then add-in the not-unreasonable noises from an 803 'box with 86,000 miles under its belt, and a noisy environment is the result! Could be worse, I guess!