Are you saying the account is used for both business and private expenses -or- that you’re looking to claim on mortgage interest as an expense for your business (e.g. you might be using part of a house for your business)?
If the former, then that’s a bit of a mess. You’ll need to split transactions across personal vs. business. I’d recommend a move to a seperate current account (and even credit card) to run your business expenses in the future.
If the latter – then that’s quite a common scenario. Your accountant will work out how to apportion interest expenses. It’ll most likely be based on portion of property used for the business – but check with your accountant. S/he’ll absolutely know how to take care of this.