Managed to solve the cable slippage through the pinch bolt using
@Gunk 's tip.
Although part of the problem was that the cable had become so chewed, that it just wasn't staying put.
So I stripped out the rear gear cable from the Emmelle and used that one instead. It's a new cable btw, because I'd fitted it just before I stopped riding that bike to try and solve shifting issues. Although that's got friction thumb shifters, so a totally different kettle of fish...
The chewed cable isn't wasted - it can be cut down and used on the front of the Emmelle when I start working on getting that back to a rideable state prior to moving it on.
Still need to work out where the tension on the RD cable really needs to be, but I can't quite finish that off until I'd got the chain on. So on went the chain without any problems (gawd, that's a wiggly, slippery thing to manhandle, but thank goodness for zip ties!), although my hands were too sore from sorting the cable to lock out the quick link completely. I will revisit that in the morning; there's a good hack with rear brake and drive side pedal, but for that, I need to fit the pedals...
The real bonus though, is that the chain runs nice and smoothly on the big ring - small sprocket combo. There's no fouling on the FD cage and there's enough clearance between chain and frame thanks to that judiciously-placed washer. One happy Reynard.
Even better, the bars and seatpost I want are now back in stock at SJS, so I'll be flashing the plastic later. I'm going for a 25.4mm post in the same brand. Given that the previous one came out as a bit oversize when measured, I'm working on the premise that I'll get a good fit this time.
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