By the sounds of it, you are fairly close to 8 weeks from your first approach to BA? If so, you can consider going to CEDR on the basis of unfair treatment. It's likely BA will try and say it's somehow not covered in the CEDR regulations, but at this point you should do your best estimate of what the refund should look like, indeed ping the whole excess spend on your BA claim, and see what happens. BA will then have a strong incentive to resolve this since in fact Aer Lingus is on the hook for some of this. The other option is to go to the Sheriff with a small claims court case against both airlines jointly. This would be for the whole claim and bear in mind your claim amount can only be knocked downwards, not upwards, so check everything carefully.