Got no further e mail as promised by Royal Mail, but as the item was shown in USPS as held at a facility here in the UK, twice removed from Heathrow, I decided to click the tracking details on the e mail link shown in my first post, where I found out the customs duty was £15.42, which was less than 20%, but more than 5% - so, no idea how it was calculated as there was no breakdown of the cost. Tried to pay over the RM website, but it failed to work.
Anyway there was a card pushed through the door when I got back from work last night, saying the item was being held at the local post office. So I decided to pick up in person and paid cash. I would have asked the girl at the counter how it was calculated, but the pick up point is at the back of the post office, it's all open and it was pissing down with rain, so left it.
Well worth the hassle as it's a really nice coin, but that customs duty is a bit of a mystery.
Apologies for rambling on.