Nina,
If you missed 5 payments at £750 the mortgage balance would go up by £3750 plus interest so roughly your balance would be around £135k so the extra £3750 you have in the bank will have been cancelled out by the increase in mortgage balance. You need to realistically consider what you can sell your flat for, take into account all moving costs (solicitors, estate agents, stamp duty etc), then work out what sort of deposit this will leave you to put down on your new property. If this is less than 10% of the intended purchase price you may struggle to finance your move. This may not be what you want to hear but alot of people are in the same position at the moment.