Definitely worth writing in - doesn't hurt, does it? But I'd also PM Lyndsay with your account details, booking ref and details of the cancelled trip, etc. as she may be able to sort something out faster than the usual channels. I think it would be very unfair of BA to not promote you since you fully intended to fly then and qualify but couldn't because of the chaos. Other threads here report they're honouring the "Fly X flights in ManyLettersOfTheAlphabet booking classes, get XX,000 Miles" bonus for anyone who was meant to qualify for that during the disruptions, I don't see why this should be different really.

Good luck...although don't expect much from the welcome pack when it arrives as mine did the other day! A card, 2 luggage tags, a letter, a stupid Avis voucher and a benefits book which is almost the same as the Silver one I got a 3 years ago (including listing LGW South as a lounge )

If nothing else, it'd be nice to try and get them to upgrade your account before you next fly for the benefits...but more importantly, it will mean you don't lose the tier points towards requalification on the next trip which should have been in your new membership year...

Phil