Thanks for all the advice, everyone. Just back!

I used West Ruislip in the end, which still had some spaces left around
8.00am. Not many but a few. I was impressed that there was an
attendant selling the day tickets, too. No need for the pocketful of
coins I was lugging around, after all!

(Incidentally I must e-mail LUL; the Station Assistant at West Ruislip
was one of the friendliest people I've ever encountered!)

Thanks again to all who made suggestions for the advice.
--
Ian Jelf, MITG, Birmingham, UK
Registered "Blue Badge" Tourist Guide for
London & the Heart of England
http://www.bluebadge.demon.co.uk