Hi Victoria, sorry for your problem. Go back over your bills and write down the actual and estimated meter readings and dates. Then see if there is a pattern of change since the smart meters. Drawing a time graph would help. If you are convinced the readings have increased then pay the £50 for a check meter, it would save you in the long run and I guess they would refund the £50 if they are proved at fault.
Smart meters really don’t measure any different to conventional ones, the problem is that they have not been sending data which has led to estimates. Have you sent in your own manual readings at alll, it would help. Good luck and keep us updated.