Straight C&P from my thread on the GC forums :

Like caching? Like trigs? Many caches are hidden near to trig points, but it's not easy to find them without a lot of searching around. You can claim two finds for each of these special types of hide - one on Geocaching.com and one on http://www.trigpointinguk.com/

If you were in the US doing this sort of thing, you could of course log their equivalent - the Benchmark - on GC.com - another thread entirely which can be discussed here or here

Anyways, I've compiled a bookmark list, and there's just over 500 at present. I've had to split the list in two, with the first list having 498 waypoints in it, and the second having 21 or so :
2 for 1 : Caches At Trigs which covers all of England and Wales.
2 for 1 : More Caches At Trigs which covers the extreme N of Scotland and any new ones that crop up.

The bookmark lists show strictly all Trad, Letterbox Hybrid or Earth Caches that are 0.1 miles or less from a Trig point.

The latter list will be added to as more caches near trigs get hidden.

Why bother? Well, as stated, some might wish to combine their two hobbys, or at least be a bit curious. Also, as many trigs are of course situated at prominent viewpoints, some might consider a cache near a trig to be one well worth hunting down (for a milestone, perhaps).