@Glos Guy It went from 17.5% to 20% I think in Osbournes 2010 emergency budget and not that long before that from 15% to 17.5% again if I remember right it was all so Osbourne.
How I understand it-any deposit paid is charged at the current VAT rate. any balance owed is paid at the current VAT rate at the time of taking ownership of the goods. Any credit taken out is also paid at what ever the VAT rate at the time of your first payment.