More money to the EC?

So, because our economy is doing well we owe the EC billions more? This seems unfair and plays right into the hands of those in the UK who are anti-Europe. The question is did we agree to this before it all happened? I suspect we are moaning now, but this clause has been in place for years!

See http://www.bbc.co.uk/news/uk-politics-29754168 .  UK PM David Cameron says we are not paying next month, but what's the betting we soon have to pay up?

Extract from the BBC website:

David Cameron has angrily insisted the UK will not pay £1.7bn being demanded by the European Union.

"If people think I am paying that bill on 1 December, they have another think coming," the prime minister said in Brussels. "It is not going to happen."
But Commission President Jose Manuel Barroso said the demand should "not have come as a surprise" to the UK.
He said it was made under a system agreed by all the member states and based on data provided by them.
EU finance ministers have agreed to the UK's request for emergency talks about the top-up payment, which would add about a fifth to the UK's annual net EU contribution of £8.6bn.