John
You're right about the day max in the charts. The 06-18 max at Pershore was -0.3, and the max from Rothamsted -1.1, so the Stonyhurst max must have been well above freezing to have thrown the made the max positive, and in an easterly that looks very unlikely.
I think they work out the mean max and min by doing a calculation something like this:- CET = ((2 x Rothamsted) + (1 x Pershore) + (1 x Stonyhurst)) /4
But it's the 09-09 extreme max and not just the 06-18 max that you get in a SYNOP.
Bruce.