After I finding
Squirt's Sennheiser intermittency repair instructions, I decided to give it a try, and I'm happy to report that the cable plugs now work without any problems at all. After the successful repair, I immediately placed an order for a HD650 cable and a replacement headband pad. I'm not sure if I really would like to use the Senn pad though, since that beyer pad looks very nice. Perhaps if I paint the headband black....

I did a quick google search of "Krylon Fusion" and uk, and I found this page.
http://www.caswelleurope.co.uk/krylon_fusion.htm
Another plastic paint product I know of is Plasti-Kote Vinyl Dye. That seems to be quite popular among PC case modders. Unfortunately, I don't know anything about its availability in the UK.