By Ruby Flanagan Brits could be paying an “unnecessary” extra £3,000 on their mortgage by making this one simple mistake. Homeowners who let their mortgage deal slip onto a higher standard variable rate (SVR) rate when their fixed deal comes to an end can be adding thousands extra onto their repayments. This deal is typically more expensive and it is down to the lender to decide what the rate is. According to research conducted by personal finance comparison site finder.com, almost a third of homeowners (31%) have let their mortgage rates revert to a higher rate for at least one month after th…