I've dug out some old notebooks, which so far have reminded me of a couple more destinations. My list of Mystery Tours is now
Chester
Margate
Southport
Somewhere near the Bluebell Railway
Norwich- think this was a Mystery Train, but it might have been a Norwich excursion, advertised as such.
Speaking of estimating speed, I bought a second hand railway book a few years ago and subsequently found a tiny Ian Allan Locospotters Club 'Members Reference Book nestling between two pages. It is 3 inches x 4.5 inches, 16 pages, a mixture of info about the club and advice to new spotters- including a warning to never enter railway premises without permission. I'm sure that advice was always rigourously adhered to... On the back cover is a table that converts seconds per quarter mile into mph (albeit misprinted as mhp in one column), from 30 seconds per quarter down to 10, in 1 mph increments. I was never a member, but this was a pleasing find nevertheless.