Dad called earlier to say that he tried extending the spring and it seems better - it doesn't seem to jump when the ISCV is connected any more and started from cold around 1400rpm, dropping down to something approaching normal, although it was hunting a little bit, it was only very slight, not the 200-400rpm surging that it was doing before.

We're both not convinced that it's completely solved, but it certainly sounds like a noticeable improvement. Still not a huge difference on the idle bypass screw, so we'll see if the new one fixes that one.

It's definitely looking like this engine would have had considerable running/idling problems and the previous owner has certainly had to bodge things to get it running.