Grapes vines need a well ventilated area with lots of air circulation, so growing grapes in a conservatory is probably not an ideal place. If the leaves have a white powdery substance on them then it's possibly powdery mildew that's causing the problem. This will spread to the fruit and cause the grapes to rot.

Apart from opening up your conservatory to allow lots of air to circulate, the only other cure that I'm aware of is to use a copper sulphate spray.

Echo above. Maybe one of those pedestal fans oceolating around might help a bit.