Thanks Grahameo. I know the back bank of plugs are older than front, as previous owner confessed he'd changed the front and then chickened out. However, his plug change was unnecessary, as all six were done at 82,000 miles and I bought the car at 96,000. It's done 124,000 now. As I recall, plug changes are at 72,000 mile intervals, so mine should be okay for another 30,000. Maybe I'll have a quick look at the fronts at the weekend. If I do choose to change them, sparkplugs.co.uk have NGK Iridium at £6 each plus VAT at the moment, which seems pretty good. I've done them on a 24V 164 before, so I pretty much know what's involved.
Italian tune up might be a problem as it's now officially off-road pending repairs. I guess I could take a scenic route to the test centre when I rebook it.