Trigeminal neuralgia 6 mth. On carbamazepine 300mg tds. Still on&off symptoms. Does switching to lyrica (pregabalin) or oxcarbazepine help? Or decompression surgery?

Go higher. Carbamazepine (tegretol) is still the drug of choice. If no side effects and blood tests (cbc, liver functions) are within normal limits, increase Carbamazepine up to 1200mg/day or higher. Before switching to oxcarb try another option by gradually adding up to 10 mg of Baclofen tid. Rec:do not self-medicate. Always consult with your physician before changing any protocol.
Surgery if meds fail. Speaking specifically to the neurosurgery issue. The microvascular decompression is a well established technique that has a good track record at centers that do a lot of them. But surgery has its risks that must be taken into account. The basis of the surgery is protecting the trigeminal nerve from a prominent vessel that "pulsates" against it.