Most dealers in UK seem to keep it in stock, so it was overnight post. I made the mistake of ordering through a local shop (see related post from a few weeks ago, probably in 'Dealers' forum). I should have ordered direct from Motomecca or similar.
I've looked up the other post, here. The seal is probably quite low cost, though I paid nearly £13 for it.
When I got the new seal I took it to a bearing factor to get a part number for future reference. He couldn't get this particular seal as it has a ribbed outer side. The part that he had listed has a smooth side. I think he said that he didn't see why it would need the ribs. When I took the old seal out, it had a smooth side anyway – so it would be worth checking at a local bearing factor – if you've got the time to keep the bike off the road while you take the seal out and check this.