OK, I found the problem.
I used a mobile crane to lift off the lid. *All* the electro-mechanical contents are bolted to the underside of the lid on this model, so you can actually remove the complete workings from their oil bath with no difficulty (assuming you have some means of handling the weight involved).
The fault was immediately apparent. This type of welder has course and fine amperage adjustments as detailed elsewhere in this thread. The contacts of the course adjustment were virtually mint in their almost Victorian splendor. The fault was with the fine adjustment. Both adjustments rely on exactly the same system: transformer tappings determined by the movement of pointers described elsewhere on this thread. On the business end of the pointers' shafts down below the oil level, there's a contact in the form of a copper blade about 2mm thick which engages between the slot of a four-way range of springy copper receptacles. On the fine adjustment described, there is unmistakable evidence of heavy arching and charring during previous operation which to fix up goes beyond what a mere clean-up and re-profile would achieve.
I want to rescue this beast. There's nothing like an oil-cooled arc welder for tackling the real heavy jobs and despite their weight, you just can't beat 'em. So the question now is.... - please take note those here that dip into threads without reading the background first - .... where in the UK can I get oil-cooled arc welder spares, or if not that, then 1mm and 2mm thick copper sheet I can trim, bend and fit myself?
cheers.