Help with re-routing

seasonedtraveller

Would be grateful if someone could advise on a re-routing.
My return BA flight was from St Petersburg to LHR. Arriving at St Petersburg (Russia) was told that flight was cancelled. Was re-routed. Had to go from St Petersburg to Tallin (Estonia), Estonia to Oslo (Norway), then Oslo - Heathrow, arriving at LHR some 7 hours late.
Asked for refreshment vouchers at Oslo but was told that as the problem originated in St Petersburg they would not provide any.
BA have offered me £200 (there are two of us), in vouchers which I have rejected (as have read not to accept vouchers). Its now with the CAA, but wondering if I should accept the £200 voucher offer.
Here is response from BA:

Your claim for compensation has been refused because BA0879 on 14July 2013 was cancelled due to circumstances outside British Airways' controlresulting in insufficient cabin crew to operate the service which prevented theaircraft operating as scheduled. Under EU legislation, British Airways is notliable for a compensation payment in this situation.

Vauban

Insufficient cabin crew is most unlikely to be an extraordinary circumstance, and compensation is therefore payable. BA have generally behaved reasonably on 261/04, so perhaps worth one more letter - with NBA wording - asking what exactly were the circumstances the resulted in insufficient cabin crew.