Broadly speaking, they're mid-platform markers. Most (?) of the through platforms are provided with one, double sided - a black cross on a white background - with the length of platform either side of each marker being numbered differently (odd numbered platforms to the north/east, even numbered platforms to the south/west). Trains signalled into the platform (in either direction) can be routed either to the mid-platform marker or to the signal at the far end of the platform, the only difference to the driver being the route indication (e.g. arriving from the north, an even number indicates authority to proceed along the full length of the platform, an odd number only as far as the mid-platform marker). It'll only be a single yellow even if the route's set right to the far end (unless, presumably, there's a route set beyond), or a position light if any part of the platform is occupied (even if the route's only set to the marker, and the line is clear that far). If a train is signalled to the marker but subsequently needs to pass it, verbal authority needs to be given.
That's my understanding at least - a signalling curiosity indeed!
Thanks for this - and the following reply about the markers
So - this is not actually a signal but a "marker". Is that correct please?
Also - is it illuminated - all the time, part time - or...?
Thanks again
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But can a driver treat an OFF indicator as confirmation of a proceed aspect? If not then the original question was whether a banner would show a proceed aspect if the signal it repeated was displaying a subsidiary aspect. May not be relevant for a train approaching, as the approach controls would probably hold the signal at danger until after the train had passed the banner anyway, but what about a short train stopping in a long curved platform with no view of the starter?
As I understand the original question about repeaters and following through to this post...
1. Drivers work on the signals they see as they see them - strictly in accordance with their
route knowledge, the train they are working and the environmental conditions at the time.
2. A platform repeater is
not a signal - it is there to assist platform staff and guards when the conditions of the platform would prevent them from having a clear view of the signal that it applies to.
3. Due to the nature of 2 a platform repeater could provide the platform staff and guards with an "OFF" indication at any time that the main signal (primary aspect) that it refers to
or any position light subsidiary or shunt signal shows a "Proceed" aspect.
Therefore a platform repeater could show "OFF" for a G, Y, YY , "Draw Ahead", "shunt (to siding via facing connection) and any of the older named subsidiary signal designations.
If a Driver were to approach the platform repeater when the signal it repeated was displaying a position light indication at least three things should apply... (a) it is not a Banner Repeater - so it isn't a signal and doesn't give the same information/degree of safety that a signal provides (including a Banner Repeater) - so, basically, he/she should ignore it.
(b) the previous signal that was past would have been at most a single yellow (There's a small chance that it would have been a position light - probably a shunt)
(c) he/she should know what the existing movement is doing and booked to do next - and if he/she doesn't (potentially due to a change in working) then he/she should be proceeding with caution anyway.
If the "OFF" indication were noticed by an approaching driver he/she would not leap to a conclusion that the signal referred to was somehow suddenly showing a green aspect.
4. A Banner Repeater Signal, as distinct from a platform repeater, will repeat the main aspect and not a position light. Both the main aspect and the Banner Repeater are Running Signals which will be obeyed (worked to) by the footplate crew and guard.
So - we can have a situation where a train will approach the signal with the position light... It will have past a signal showing one yellow, it will be slowing to a dead stop or approach control speed - ready to stop dead - and both the footplate crew and guard will have observed these conditions - and (again) should know what to expect from the booked working or additional (verbal) instruction. A guard at or near the back of a long train may consequently get a situation in which a train moves from standing or a very slow moving train draws forward while the Banner Repeater is "ON"... He/she should, however, promptly see the main aspect - showing red - and the position light providing its "non-running" proceed indication. If the position light was not illuminated the guard would apply his/her brake control and bring the movement to a stand.
Again - route knowledge, timetable knowledge or additional instruction would apply.
I hope this helps
I'm trying to recall where I've seen a little LED Banner Repeater...??? 
Not in my day! 
Officially we had Ground Signals, shunts and three types of subsidiary - and do not mix them up!