I like the speculation on hours and times and how people look at freight world....I hardly worked nights and not much before 5am either, though we did lodge......
I left the freight side a few years back, I didn't work for GB but we used their messrooms in some locations. I'm actually out with a GB guy Saturday, but he's not based at Wembley or Ferme Park
The 12 hours and start/finish times....Its pending what depot you go to and what work that is at that depot, this would be something I would of asked at the interview....I'm not saying your don't work 12 hours in some cases, but there are jobs less than 12 hours....You do have AV, which is non timed spare, but I think they have datum times to sign you on, but if you get AV with a star, that's ballast spare turn....Someone will correct me if I'm wrong.
I'm not sure of the conditions now, I think you can have PNB's in weird locations, like a service station, you can drive a car/van in your own time to a ballast, but they sort that out.
Wembley depot mainly do the sleeper work, Intermodal traffic and stone etc etc, but its pending where you sign if you are a qualified driver moving over. I'm not sure if you would ever sign Preston for the sleeper work. If they have enough drivers for that work, you may never get to learn the route. Again that's something you should of asked at the interview maybe, also ask for a roster to look at, if that's the depot in question.
Wembley I think is the better depot.
I can't comment on Ferme Park, but it used to be more of a ballast/aggregate depot.
The best thing I would do, is to go through your aslef diary and look for GB representatives and asked for some info. The guys are pretty much on the ball and all approachable.
Also look into the pension, unless your ex BR, check the pension you would go into.
Like I always say...Freight is not for the faint hearted...lol, I was glad they invented ipads!!!!!!!
Good luck