Type of thermostatic shower valve

[blazindata]

Hi. My shower temp is too hot. I've taken the valve out to clean. If this doesn't work I would like to know what type of shower thermostatic valve I need. It's held in by a brass screw in "o ring". It then pulls out. There's no grub screw. What is this valve called please for me to search a replacement? There is no markings on the mixer to tell me the make. Thanks.