Thanks for replying.
I actually updated directly via my account settings at https://www.netflix.com/youraccount
Interestingly I tried again earlier today and an hour later I am still seeing my account as Premium.
I am wondering if this is a timing issue. Yesterday it was at 17:51 that I did the first attempt and got the 'Your plan change has been declined' message at 17:53.
Today I did it at 10:05 and not only is it still set at Premium, I also got a confirmation message from Virgin 3 minutes later. This makes me think that this can only be done during working hours.
At the moment all is sweetness and light and I am a happy bunny