The rest of the story is that by the time I had done the (normal) 10 minutes walk, I could hardly stand!
So, it looks like a week off work - and I don't get paid, which is not good for the bank balance. Oh well, it could have been a worse injury, so I am still thankful not to have ripped my Achilles tendon.
I have physio on Friday, so will have to wait and see what they think.
Now must go to phone my office to tell them I am back home ... communication is not always great there!