Happy New Year everyone!

I checked the compensation thread but it seems to be mostly related to delays and cancellations. I also tried searching but am coming up blank. If this has already been answered then please excuse me.

My partner was flying in from the US to LHR and then connecting up to EDI. Her flight was delayed 5 hours departing the US due to the BA IT issues that occurred 19th - 20th December. She therefore missed her connection up to EDI. BA did get her on a flight a few hours later (which is fine) but upon arrival in EDI her bag was missing. It took BA a full week until after Christmas to reunite her with her bag. It seems to me like BA are solely responsible here?

Obviously she had to purchase various replacement items which she didn't feel were excessive but considered essential. We actually waited several days before replacing but given the length of time it took to get her bag back she had to purchase several items of clothing, cosmetics etc. Anyway, the total came to around £350 and we submitted the claim to BA with all receipts included. BA have now responded saying they are compensating only £42.

Obviously this isn't acceptable to her but I am not sure how to word the response to BA saying this? Is there anything specific I should be quoting or does anyone have any advice? I believe this is handled by the Montreal Convention?