There was only one 57 reg Solo SR which was the very first one made, which appears to have been scrapped three years ago. Anyway, the system is the same on all diesel Solos - the vents at the rear of the passenger saloon are for heating and very effective in the winter if everything is connected properly. They have a negligible effect on cooling the engine down if it is running hot, having the demisters on at the front is much more effective.
I think the question that needs to be asked is - do you want the bus to stop moving for 10-15 minutes while it cools down, or do you want it to keep going? The railways don't keep going at full speed when it's unusually hot, so why would you expect a bus to?