Mortgage switching - paying interest on both!

First Off Happy New Year to everyone!

Has anyone experienced the following problem before? We are switching our mortgage from AIB to Ulster Bank and after lengthy delays we finally got word from Ulster Bank before Christmas (21st) that they have released the money. However, probably due to Christmas holidays, the money is still not in our account - it's probably with the solicitors still. But we then received a second letter from Ulster Bank stating that we are being charged €240 in interest for the mortgage for the month of December - which means that as our AIB mortgage has not yet been paid off in full we are paying interest on two mortgages! Surely this is not fair on us - but who is culpable? Can Ulster Bank charge us interest as soon as they release the money, not when we actually receive it? Or is it the solicitors we should complain to for not transferring the money to our account and closing off our AIB mortgage in time?

Just to follow up on this, we contacted the solicitors and explained the situation and they gave us a choice of either paying the interest on both and probably have the mortgages switched over to the Ulster Bank one by this Friday, or else they could cancel the issued cheque from Ulster Bank and get them to reissue the cheque in which case the interest charged to us would be refunded and by the time the cheque is reissued they should have a final balance from AIB on our AIB mortgage and so be able to switch the mortgages that day, thus avoiding us being charged interest on both. The only downside of the latter option was that it would delay the switch by about 5-7 working days but we went for this as it meant we wouldn't be charged the €240+ interest on our Ulster Bank mortgage whilst we were still paying our AIB mortgage.
I would still be interested to hear from anyone else that went through this process before to know how they fared.