Sorry for the poor photo, and it's definitely a heat detector, but I'm trying to determine if this there are any differences between Pt1 and Pt6 detectors and, if so, where this one fits?

Also, the manufacturer would be helpful because, although I've seen loads of them in situ, I can't find photos of them online.

Thank you ?

It's a Rafiki/Fike Multisensor. It's a combined smoke/heat detector that can be set to smoke only, heat only or multisensor smoke/heat and often includes an integral sounder.

It forms part of either a Rafiki/Fike conventional twin wire fire alarm system or one of their addressable systems - both Part 1 Commercial systems with control & indicating equipment. 

Very comprehensive, thank you.

Could this detector be  used in a Part 6 system, or would that be considered 'overkill'?

Thanks again.

If you are installing a Grade A system, yes (usually required in HMO's and recommended for very, very big houses)

Thank you ?