Purple, I think you need to do some basic sums to work out what sort of return you might get on this property.
I believe that the usual formula for calculating your gross 'yield' is the monthly rent x 12 divided by the purchase price then x 100 to get the percentage. So in your case, this would be 1200 x 12 = 14400/310000 = 0.046 = 4.6%.
This sounds respectable but it is the gross yield so it doesn't take into account any associated costs of owning the property e.g. mortgage repayments, insurance, ground rent, service charge, ongoing maintenance and repairs.
Then there are also the costs of finding and putting a tenant into the property e.g advertising for tenants, gas check, inventory, putting deposits into a deposit scheme, cleaning etc.
So to get your net or true yield, you need to add up all these estimated costs and deduct them from your annual rent figure. Mortgage alone will probably swallow up very approximately £400-500 a month.
Then do the yield calculation again using the actual annual rental figure to see what your net yield will be.
I think that's all correct but be warned that maths is not my strong suit and someone else might come along and correct me!
Once the flat is let, don't forget that as well as your outgoings you will have to save money every month for the taxman, although some expenditure is tax deductible.
Basically, BTL does take up a lot of time and money. It's not a recipe to get rich quick, that's for sure.