Stopped at Avon Forest, after eight miles, and found the pressure was now 2.5psi higher than when I left home, because the tyre had warmed up. Whether it should really have been higher still, I didn't know, but at least it had gone up, not down. Thus encouraged I joined other club members on a ride to Sparkford via Salisbury and Shaftesbury. It was lovely to be out and about on uncrowded roads, plenty of overtakes to be had, fingers nice and toasty.
75 miles later, after a cuppa at the Haynes Motor Museum, the tyre pressure was a mere 2.5psi below normal. I decided this was OK, re-pumped it up, and headed for home via Sherborne, Sturminster Newton, and Blandford.
The puncture's in the centre of the tread and looks as though it might be repairable. On the other hand, the tyre was due for replacement within the next 1,000 miles or so anyway – was planning to get it done at the next service. Glad to know it'll probably get me to work tomorrow. I'll see whether the shop will repair it but contact the mobile tyre replacement people as well.
ETA Nice pic by Roger - thanks!