ok, well 2 more calls made to explain this rule. First agent said that because there is longer than 72 hours between my flight on Dec 17th and the one on Jan 4th it was not allowed. This is a deal desk rule - I pushed him on it and said that it was not acceptable to have internal, unpublished rules (and probably not legal either) and he said it was to do with people doing RTW tickets which clearly, I am not. Anyway, he went off to check as I said I wanted to see the actual written rule that stated about this 72 hours and then we got cut off.

Then next agent said it was not allowed - so I pressed her on why not and she said it was because I had to return to the same destination. I mentioned that this is not in the rules anywhere I can see and she went off to check and basically wouldn't budge. I asked her to copy me or direct me the T&C's that stated that I had to start and finish in the same place. after much holding she came back and told me that it was not BA but was the IATA fare construction rules and said I needed to read section 15.15 which is to do with ticketing of mileage tickets (not specifically BA but these rules apply to all airlines). Effectively LHR is in Zone one and Oslo is in Zone 2 so it is not possible to use frequent flyer miles to start a journey in zone one that doesn't end in zone one (it can have multiple segments). So, my request for the add on was denied due to IATA.

I find this explanation pretty hard to believe but maybe its true ??