In reply to adstapleton:
I had the same too, but when I spoke to my insurance company, they said the insurance would be void if I hadn't declared it as buy-to-let to the mortgage company. I thnk they said it was becasue the mortgage company has an interest in the house.
Speak to your mortgage company as it may not be a problem to convert. The mortgage on my flat (Halifax) converted with no extra penalties and I then transferred to a buy-to-let mortgage later.
Interestingly, when we moved abroard, I wanted to do the same on my house. Initially they (ING) wanted to charge me an extra 2%, but I then found out that if you tell them you have moved away for a fixed short term period (up to two years) and that you had to move because of work, they view it favourably and didn't charge me the premium.