Haven't seen NU 7 day cover on ebay for ages! So not sure about that..
Otherwise, depending on your age, try the following:
www.dayinsure.com
(over 21)
www.ecarinsurance.co.uk
(Short Term Policy - ages 25 & over)
Or, if you have your own insurance policy:
Phone up your insurer to check if you have "Drive other Cars cover" - if you do, then providing your bf's car is insured under it's own policy, you can drive it legally. I say that it must be insured under it's own policy, because if it's not, then the second you step out of it, it will become an uninsured car.
Lastly, if he has cover on the car, couldn't he just phone up his insurance company and get you added on to his policy for XX days?