Looks like a (fairly old) double-check non-return valve to me. I can't see the part number from the photo but Westco are still in business, try asking them.
I speculate that the inlet temperature is getting too high, say 30°C but check the shower spec, on hot days day so the shower is cutting out. Would probably be okay if you could run the few litres of tepid standing water through before turning the shower heater on. Is the shower on the end of a long branch or would running a bath/basin tap or flushing a toilet be a possible method of flushing the warm water out?