Why aren't they settling?
They don't seem to be a licensed financial institution, which means they shouldn't be sitting on your money at all. You should be getting the settlements from the acquirer. If they are the ones receiving settlement, you are dealing with some very dodgy people operating outside the boundaries of the laws and regulations.
If you are unable to reach an amicable solution with them, your options are probably limited to litigation in UK and possibly India. Sometimes, well-crafted threats of litigation are enough to make someone soften their stance. You can try approaching the FCA but it's unlikely they'll take action.
If you can find out who the actual processor is behind the scenes, you can try to contact them but they most likely won't listen since they have no idea who you are and have no relationship with you.
It's not an easy spot to be in. Hope it works out.