Thank you for your help - customer said they'd look for their old receipts. Fairly sure it's not from a shed (luckily!)
I'm sure I can remove it (will have to if it needs replacing!), but again I'm reluctant just at the moment. As they're neighbours (and good customers!) I said I'd have a go at finding a cartridge. If it needs replacing then they'll probably want the basin mixer replaced too.
Why don't these bloomin' manufacturers stick to good old fashioned washers? These cartridges are a pain in the neck as replacements are expensive and if you order the wrong one the plumbing merchants won't take them back. And it means much more time spent repairing/shopping/ordering, etc. In contrast it took me about 5 minutes to sort their outside tap out with a small rubber circle.