could anybody identify the make/model of this PIR? It was installed with the house alarm in 1994.

It a gardtec something or other.Long since passed its change by date.

Yea, I know. Relay inside of it is very loud. You hear it click

Coincidence ? The pir's at my property were installed in 1994. However, I see no reason to change them since they are working perfectly. Pick-up patterns, walk tests etc. confirm. Maintenance is limited to cleaning the cover, with a quick check inside (all holes are sealed anyway) and wiping the fresnel occasionally.Probably someone can tell me why they might be, or are, in urgent need of changing.  

Like any electrical component the mechanical parts (relay) has a finite life, imagine how many times this will have operated, so ime for the sake of a few £'s id be looking to replace.

Yes, I thought they worked perfectly. The only reason I can think of is the relay inside is reaching the end of it's working life. Apart from that and the fact they have gone yellow I can't see why they need changing.

Is the ADE  Activ8 any good?

They are quads from what I can gather. It is worth checking if any structural or other alterations have taken place since 94, also the addition of pets etc. (you know all that already, why am I writing it ?). Provided very little has changed I see no reason to go beyond a standard pir replacement (others would disagree), although these days there is a tendency toward quads, be they ADE or any other make.  

Thanks, Might get Pyronix

Nothing wrong with Pyronix

Will use Pyronix or ADE then since they are both good companies.