I just happened to notice this post today...
1) Points within the yard are hand operated. There are some traditional point levers, but the majority in this part of the yard appear to be American-style "switch levers" with a sort of white hoop instead of a lever, the type of thing seen here (in the USA) -
https://www.flickr.com/photos/dldruss/52273434059/
2) The only signals are located at the exits from the fans of sidings onto the running lines, as seen on the Open Train Times signalling map:
https://www.opentraintimes.com/maps/signalling/westbury#T_WSTBRYW
If I'm looking at the map correctly, 702 is the ground position light mounted on a pole (or whatever the technical name is for a GPL that's not on the ground) controlling the exit from the spoil tip sidings.
704 is the same type of signal, also on a pole, controlling the exit from the wagon repair sidings and sidings 12 to 7
3) The shed is marked as an "OTM Shed" on my Trackmaps (i.e. for On-Track machines) and labelled as the "Elephant House."
This is a decent shot of the exterior:
https://www.flickr.com/photos/93293618@N05/53109842233/
And interior, with inspection pit and some stairs / scaffolding visible:
https://www.flickr.com/photos/90265707@N03/52004067733/
I assume it used to be used for track machines (or elephants) and that Colas have taken it over for loco servicing. The pit would be useful for this - depending on the loco type and the work being done, certain routine exams or repairs can only be done where there is a pit for accessing the underside of the loco.
The wagon maintenance / repair area is on concrete hardstanding alongside. There are storage containers alongside but not much fixed equipment.
In this photo there are wheelsets adjacent to the siding, and the red machine is possibly a lift truck used for wheelset changes:
https://railphotoanthology.weebly.com/uploads/7/0/6/6/7066978/dji-0155ed_orig.jpg
4) The main reason for answering is because I was recently looking at some photos of Westbury yard on this RailPhotoAnthology page (not my website but I recommend having a look). I think the drone shots here will answer most of your questions:
https://railphotoanthology.weebly.com/westbury-yard.html
Judging from these photos, locos and track machines also stable in the Up Yard. There are couple of YouTube videos on that webpage too, if you click through to watch them on YouTube itself (RailPhotoAnthology channel) the quality is better and it gives a good overview of the yard by drone, including the area you are interested in near the wagon repair sidings.
