The water temperature can be considered as an average of the temperature throughout the day (and night). However, the heating effect of direct sun also needs to be factored in, and for the UK, this can be considered as an increase at the peak time of 20C (approx). Other times of reduced sun would need to be adjusted pro rata (with some estimation).
As a result, with no sun at all, typical temperatures now could average at 20C. Adding the sun adjustment can raise it well beyond 25C, perhaps even 30C (as an average remember). The temperture excursion between morning and evening will only amount to a few degrees, hence your warm butt !
I hope my postulation has not caused too many headaches !
