I should have got back to this before now (apologies).
Buying from the USA worked out really well. I couldn't find a used gearbox actuator on a European eBay site where I was happy about the provenance of the donor car, i.e. How many miles it might have done. Consequently I ended up buying from a site in the USA ('Euro Chop Shop' in California) where it was more transparent about the mileage. I transferred the money for the part on a Friday and it arrived the following Friday - prompt or what!
Anyway, I ended up paying about 20% VAT + import duty (£15) on what FedEx had asked me to pay on what I thought was a 'fair price' for the part. In other words, they didn't actually ask me to state what I'd paid for the part, only what I thought was a fair price...
I guess this type of experience may depend on who you deal with, but for me it was wholly positive.