I have a paid-up Life Policy that matures in 2015 for £4300 if it matures at the rate of 3.75%
I can surrender the policy now to the original insurance company for £3900

I have been told that I can sell it for a larger amount to another company. Is this true?
If this is true,how do I find a reputable company who won't rip me off?

I was only aware that you can transfer a life policy or do an exchange but I'm not sure you can sell a life insurance policy?
If your life insurance policy is paid up, are you just needing cash and don't need the protection anymore?
Is this a whole life policy? You can check with another company and see if you can switch to a variable life policy which may give you a better return (VULs have mutual funds).
You can check A.M. Best for ratings on insurance companies and call the most reputable ones.

The only thing you can sell (maybe) is the death benefit if your health is failing and you need the cash. There will be a substantial difference between the offer made to you and the face amount of the policy. Other than that, the only other option is a 1035 exchange to a new policy. This will trigger new fees and commissions.