Today he wants to borrow an extra £25,000 (because he is moving house and going "up market") and is going to repay the extra borrowing on the repayment method, and have that part finish at the same time as the savings plan matures and pays out. So the repayment part is set for a term of 19 years.

Typically this "part and part" method wins favour with someone who is a little concerned about the projected growth on their savings plan and its ability to pay out enough in the future. By taking the extra borrowing on the repayment method he is spreading the risk and, importantly, can go back to his lender in the future and increase the portion of his mortgage on repayment if his fears about his savings plans growth are realised.