With this tariff, do I necessarily need to sign up for the 'Recurring Payment'? If I just credit my account with £5.90 each time I want 1000 minutes/month, will this work?

My understanding of the Sipgate 1000 tariff is although there is only a 1 month minimum contract you will need to sign for recurring payments as you would with and Internet providor. You can cancel at any time though one final payment would be taken.
The only non committal method is just to credit the sipgate ordinary payg account with £10 credit which won't expire at the end of the month.