I contacted the Norfolk based fabricators. Nick retired about four years ago apparently, so Bob must have had his bumpers made before then.
Anyway I spoke with current proprietor, and he reckoned about £280 for a pair all polished up. A bit more expensive than having my originals re-chromed.
However another chap on the Isetta fan club Facebook page has found a fabricators in Northampton that will make a set for £200 all in. That's a fair price, and pretty much matches the cost of re-plating my steel originals. Apparently the fabricators make items for F1 cars, so I reckon their work must be high quality.
I await to see the results.