Rates of hotels vary on Bintan from Banyan Tree 5*to Mana Mana. You don't state which hotel was quoted to you. Send an e mail to
manamana@pacific.net.sg for the period you want and I am sure you will find the prices well below what you appear to have come up with.
Even the single occupancy rate for AMSLAM Week, Asian Windsurfing Tour (see
www.manamana.com click Amslam icon),which icludes b'fast on pre race days and full meal regimen race days, is approx US$380 or 215 Pounds - which doesnt go very far in the UK.
Good luck