The seal and calc filter I ordered turned up. Here’s what $73 gets:
Fitting was relatively trivial however I did take the liberty of giving the machine a decent clean while I was at it.
What I found interesting was the build-up that had formed in the group head where the seal was; a black sticky mess almost like tar. It appears to be residue where coffee has reacted with the rubber of the old seal to form a tar-like substance. I scraped as much of this out as possible before fitting the new seal to help the two surfaces mate properly.
Unfortunately I took the photo after I had already started so much of it has been scraped out already.
Once all was done I powered up, made a coffee and was dismayed to see the machine leaking again! Fortunately this ‘fixed’ itself after pulling a few shots but I remain unconvinced.