Oh Jesus the Horror!
Sennybridge is somewhere in Wales. Where exactly it is I can't tell you as like B A Baracus in The A Team I have a fear of the place so deep that I have to be rendered insensible to get me there. OK this isn't quite true - I'm asleep on the Four Tonner.
This being the case it may not be in Wales - and frankly from my point of view it could be anywhere. This having been said, the fact that most of it is at 45 degrees to everything else, is freezing cold, pissed wet through and desolate leads me to assume that it is in fact in Wales.
Sennybridge has the advantage of instantly creating exercise realism to aid the Character building nature of any exercise. This of course is achieved by giving every man jack the arrse by getting them wet, cold, knackered and pissed off almost immediately. If you peruse the "worst exercise in the world ever" thread you will see that all the very worst are sited in Sennybridge - this is no co-incidence.
Sennybridge offers ample barrack-style accommodation in the form of unreasonably dense forestry blocks - nighttime infiltration of which is aided by plenty of difficult-to-spot gullies, shell scrapes, and sharp branches protruding at right angles. Excavation of any sort is made easier by the notably dense network of remarkably robust roots that fill the meagre square footage between the densely-planted trees - the primary purpose of which is apparently to lengthen the preamble to getting your head down, thus shortening the duration of any rest while simultaneously ensuring it's of low quality. This second part is achieved when you discover that your shell scrape isn't quite big enough for you and your kit, unless you sleep at an uncomfortably bent angle - but this is usually immaterial as between being on stag and being too cold / pissed off to sleep there's barely time to get any shut-eye.
See also SENTA