So I've now found the wheels on the trike are not 451 but the even rarer 440 (500A) size. I can find tyres no bother (St John Street Cycles coming up trumps again - but rims - don't even get me started!

I only need one (a rear that's buckled beyond what can reasonably be done by truing) but can I find anyone IN THE WORLD that has them? Nope. I've mailed Pashley but don't expect a reply too quickly because of the upcoming break for christmas. Anyone dealt with sourcing that size before?