Using tracking I found my parcel is sitting in a depot waiting for customs charges to be paid.
On the Government website '4 slot PCI expansion' doesn't have a commodity code, however the code is likely to start with 85, the one after 84 -- Nuclear Reactors or parts thereof.
This should have a 2.5% customs charge. I'm being sent a letter so we'll see.
It seems that the nasty, foreign, European red-tape has been replaced with home-grown, sovereign, red-white-and-blue tape that, when it was pulled out of the cupboard, was found to be rather dusty and antiquated and may not be fit for purpose.