Deeply advanced and quite inadvisable fettling tonight.
Took out back wheel, very loose, spoke tensions all over the place. Try giving everything a turn to start with and one nipple split in half (I can't use my 4-sided spoke key because they're thick 2.3mm spokes). Sod this, nothing is lubricated and I don't stand a chance here.
The surplus radial front wheel is sitting there looking at me, going "No, Roger, no!". Too late. Its spokes are the right length. Dismantle rear wheel, remove sprocket and spoke guard, throw away thick spokes. Dismantle front wheel. Oil rim spoke holes and spoke threads and build decent, tight, maintainable rear wheel. 2.5 hours' work, and I also managed to fit the dynamo front wheel and get a final test ride in the dark
After my experience of refitting the rear wheel I think I'll put the Marathon Pluses on. I don't want to be messing with tab washers and SA gear chains on the road.