Recently had ultrasound which showed 31mm choc cyst and been put on 4 months of zoladex (goserelin) but should i be asking about surgery as doc didn't mention it?

Yes: I think you should know all of your options. Of course, your cyst is small, and a lot of doctors won't operate for cysts that small. A lot depends on your plans for fertility, as endometriomas can prevent women who want to be pregnant from being able to become pregnant. I would ask for another appointment to discuss all your options with your doctor and make sure you agree with the plan.