I did loads of research before booking our flights for Christmas/NewYear this year and the charter direct from Gatwick was the best option. It cost more (not as much as I thought it would) but by the time we weighed up the waiting time at Mumbai for an hours flight to Goa it was better for us.
We've been to Kerela a few times and really hate the wait at Bahrain, so the option of going direct and non-stop to Goa was a real bonus for us. It's not so much the going, it's the coming back.
If I remember, the flights to Mumbai and then Goa worked out at around £550+ each but there was a longish wait at Mumbai. The direct charter flight cost £630 each. It was around £400 cheaper to go charter with MyTravel from Gatwick than charter from Birmingham with Thomson, which is a shame as Birmingham is only half an hour away.
It's amazing how much the prices fluctuate.