Even a few years ago I may have been able to find someone who knew more answers but sadly they are all dead now.
Sad indeed, although this is how I came by it from the estate of a deceased model engineer.
I don't think it was sited outdoors as the rusting is only slight and the wood is still solid.
Yes it reminded me of the old training school layouts.
A bit more on the inter-locking;
Reversing 8 doesn't prevent setting any of the up routes, however 8 can't be reversed if 9 is reversed. Despite earlier comments I believe 8 should be up (before) 9, as drawn.
I believe 5 is at the far end of the down platform, to hold trains in the station.
Could it be the DL train splits here, an engine comes from the siding and couples up to the rear 2 carriages.
Then shunts over 15 to UB. The main part of the trains continues DL.
15 and 14 can be reversed with 10 in either position, but no 11 (fpl) required. Really there's nowhere else to go but up, so perhaps this is accepted as it is a shunting maneuvre.
5 and 6 can be reversed at the same time 15 and 14, hence the splitting train theory.
5 must be beyond 15, but 5 must not be passed if 15 is intended to be used, so they must be at least a train's length apart.
Many thanks for all the interest,