Advice on my plan please

superspudking

Im looking for a little guidance as to where I need to be in about 18 months time, where I intend to buy a 2nd property.

My current situation
Recently moved in with the in-laws and have our flat being rented out. Aiming to save about £30k over the next 18 months, add this to our long running savings account which we can access in 18 months, which should be worth another 25k. So a total of 55k in 18 months.

In 18 months time Our flat should be at a LTV of about 70% and worth £200k. Ideally we would like to keep it as a BTL and then buy a house of about £270k with a £55k deposit, 80% LTV.

Is it best to just keep things ticking over as they are? I make about £75 per month on the rented flat. Do I save it towards the house? Or over pay the mortgage? Will having a BTL have any effect on my application when I want to buy a house? Any other advice?