I have had a look around services yard - there is a big skip full of wood and stuff with an empty trolley right on top, which might be our trolley. I couldn't see anything that looked like what's missing in the skip so I talked to the yard foreman, who put me on to the services office who put me on to first response, none of whom had had anything reported. This could either mean that someone else took the stuff before (any branch of) services found them, or else that I just haven't been able to find the right branch of services to talk to. But they don't seem to be in the yard anyway.
I'll try again soon in case they just haven't been found yet.
Thanks for the tip.