What term were you considering for repaying the loan?
Depending on your lender, you may be able to top up your mortgage for the amount you are looking for and pay it off over 5 years. This way you would be availing of mortgage interest rates and still repaying your loan within 5 years. You will have to pay for a valuation report and you should also ask if there is a legal fee involved as it probably wouldn't be worth your while paying a legal fee on a loan of this size. Also, unless you already have adequate life cover you will need to increase this.
To be honest you would probably be better off going with a bank loan. Top ups are slower and there is a lot of paperwork involved - probably as much as with your original mortgage application.