I agree with AZB that you could try the British Tour operators (Thomsons, First Choice, JMC, AirTours & Cosmos), but they are a bit reluctant to sell seats due the an American Airlines agreement that Charter Companies should not sell tickets OUT of the Dominican Republic.
The above do flu out of Santo Domingo too, so get hold of the company's local office & get them to do some checking for you. You might have to settle for one weeks notice of flight or even a last minute avaiability - but it CAN be done & price used to be about ?200. If you are coming back, the round trip @ US $800 sounds like a good bet. Flight only from UK presently is ?500 PLUS!! - Grahame.