You will also need landlord's contents insurance. It's normally not much, unless youre letting out a fully furnished house. It will cover things like fridge, freezer, washing machine, carpets etc.
You will need to tell the mortgage company. There may be a small charge from them.
You should also be aware that you may not be able to get back into your flat in 6 months time. If you get a difficult tenant it can take at least 3 or 4 months after that to get them evicted. It's something to keep in mind, that you may need to find alternative accommodation until they have vacated. Also, you may find it more difficult to let out if there's no chance of the tenant's being allowed to stay on longer than 6 months.