I can see no problems with you taking out the mortgage. It might be worth putting it in writing that you are lending rather than giving them the money so that, if inheritance tax is ever being considered, you would not need to pay this tax on the repayment of the £50k. You will also need to decide which property (yours or theirs) would be used as security for the mortgage. You can also consider the alternative of being a guarantor to a mortgage in your parent's names.

My parents( both over 60) would like to move house, they currently own a property with no mortgage. The equity in this property is however not enough to buy the new property outright(200K), there would be a shortfall of around £50K. Can i take a mortgage for the shortfall ? Is there tax implications as i am already a homeowner with mortgage ? or any other alternative i should consider ?