In the end Ken had to bail due to having to get a report finished for a client in Canada.
I phoned the office at Wallasey on the off-chance that I might re-arrange our booking for another day. They couldn't have been more helpful, and Ken and I are booked in for 6th August. The £90 I'd paid online has been carried over.
Well done Wallasey!
As it turned out, a thunderstorm struck the Wirral at around 5.30 so Ken made an excellent call!