Thanks coconutty!
I had been paying £24.99 a month for what seemed like forever and had been gradually upgraded to 750K.
I followed the link but, unknown to me, they had different username and password to the ones I had recorded so it didn't work.
Surprisingly, the customer care number worked well and I was upgraded to 2Mb and also given the correct username and password! Was told that it would take 24 hours to be upgraded but, on checking later, it was done within the hour. Great result!!