I can't help with the cause or solution except to say it's still under warranty, it's obviously disappointing you've only had it 2 months and it's been back 5 times. Did you buy it privately or from a garage? if from a garage then you have more rights of redress. MG3's had an issue whereby the clutch pedal would stick down, this was caused by a fault in the master cylinder, I guess this is why they changed it. The only help I can offer is to suggest you keep on at the garage until they fix it, or threaten them with the Sale of Goods act (of which I'm no expert) if you bought it from a garage.