Thanks all for your advice. Sorry I haven't thanked you earlier, I am just able to log in and reply now!
I suppose the Embassy has to respect the rules of other Government departments, so I will mention that in the letter (although I will try once more to get a provisional date at another Registry Office). I could make a provisional booking at a Hotel - and I think that is a happy medium.
Xebec, your message is very reassuring for another reason. It seems the ECO's are reasonable at times.
So, my plan for now is to make a provisional booking for a reception, to give the registry office one more try. After all, even if they refuse us for the reason that there is insufficient evidence that a wedding will take place, I am confident that we could contest that in any subsequent appeal (not thinking negatively, just practically).