I have paid off my mortgage on my flat. I plan to remortgage it to buy another flat to let out.?

If I buy the second flat, will I have to pay tax on it even though I only have one mortgage
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Previous answer is incorrect, there is no tax free amount for letting income. You can take all the expenses into account includng mortgage interest for tax and then you pay tax at your highest rate on the profit.
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also you should keep your mortgage to a minimum on the flat you live in and max out the one you rent, mortgage interest on a BTL can be set against rental profit to reduce tax liability, mortgage interest on a property you live in is not*


*I am not an accountant please speak to one before you proceed with this!
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yes

you will have to pay tax on any extra rent over £3600 in a year

and you will have to pay capital gains tax when you sell it

sorry but as its not your main residence these rules apply