Blissbak - we did ask the reason and were told our earliest departure time was on the hour because that was our FPL departure time. I repeat - no slot.
Chevvron - thank you again for your efforts. What I now need to find is some more detail :-
1) is this purely a British procedure,
2) does the + or - time apply to off blocks time or to an airborne time?
I think this may be the crux of the problem. I am hoping someone with flow control knowledge will supply the answer and reference to this.