So, an update. Offered £12,000 for the blue Gruppe M car but that was rejected and I was told they wouldn't consider anything like that figure. I have left that car alone as a result.
I have come across two more cars that I like the look of and there is a new car on eBay that looks in good condition.
Prices are all over the shop though.
For example, this private sold Sepang Bronze eBay car https://www.ebay.com/itm/BMW-Z4M-Roadster-3-2-Extremely-Rare-Sepang-Bronze-2006/302564878454?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649 with 67,000 miles is the same price as this dealer car, with 15,000 miles https://www.autotrader.co.uk/classified/advert/201710090117220?logcode=visc&fromSA=true. Both cars are in great nick, but I can't understand how a private seller expects to see that sort of money for his car when the dealer car is lower mileage and comes with the statutory protection you get by law.
Also, this looks a nice car https://www.ebay.com/itm/2006-BMW-Z4M-3-2-Roadster-FULL-HISTORY-VIDEO-ON-WEBSITE-RARE-COLOUR/142536698444?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649 . It is £1,000 cheaper than the eBay Sepang private seller with 17,000 miles less!
I'm going to investigate the two blue dealer cars. I think I'd be happy with either of them. I wouldn't mind the Sepang car if it was more reasonably priced.