I got 3 hours in today and managed to disentangle the harness from the frame. The loom is in plastic sleeving which has gone hard/brittle and in a number of places it goes between some gussets and the headstock, the space is too small to get the connectors through the gap so these had to be cut which settles whether or not I have to re-wire it.
I originally intended to sandblast the frame but it is deeper than a Monster frame so will not fit in my converted filing cabinet blaster. In consequence, I had to use a wire brush on an electric drill and in the last two hours I managed to get about a third of the frame back to bare metal.
One little oddity The very top headrace and only the top one has very evident ball-shaped indents in it which is a bit of a bugger as I cannot, at the moment, find a source for a replacement set.