If current mortgage on your PPR is 50K and you get a further 200K loan to buy investment property then:
1. you can continue to claim TRS on the 50K and
2. you can deduct 75% of the loan interest on the 200K from your rental income when calculating your taxable rental profit
I did this a few years back and the bank (BOSI) very helpfully gave ne a new mortgage account for the investment property, even though the mortgage was taken out on my PPR, in addition to my esisting mortgage.
This made it very easy to separate the two interest amounts.