Gold cake desk

Shall I go for a tracker buy to let mortgage on my current property that I dont live in? - cheapest buy to let mortgage

I have, I leave 120k fixed rate mortgage with a permit until the end of next month can pay £ pcm 630th I currently have 495 tenants, the PCM that want to rent there for a few years pay £.
Here's the dilemma:
I will try to allow for another permit for 12 months or should I buy my mortgage so that it might be cheaper to go? If I'm making a purchase, so I go for a tracker or fixed rate? (For the interest rates stay low?)