Hi Louise, after looking at the figures, if you were to go with the Northern Rock loan over 36mths, you will end up paying �50 more than if you kept your existing Lloyds loan and would also be paying the loan for an extra 9 months so I really wouldn't advise you to go down that route for a saving of �40 per month.
If you are wanting to borrow an extra amount for a car then moving to a different lender would probably be the best way to go. I have done some quotes online for you in respect of both of your options or just paying off the exising loan and also the �6,000 loan.
Repay existing loan only:
http://www.moneyexpert.com/Search/Results.aspx ?productTypeID=2
�6000 loan:
http://www.moneyexpert.com/Search/Results.aspx ?productTypeID=2
Hopefully this helps you but if you have any questions or not sure about something that I have said, just post back again and I will check the post again shortly.