Sold phone, do I need to stop the contract

not that this is in the right forum but still. 😉
Yes you have to complete your contract. i believe that sometimes you are able to end it early, if you pay the remaining costs of the contract. e.g. if i broke a contract 10 months into it, to end it i would have to pay the remaining 2 months and any outstanding charges.
You should check your operator's website for their terms and conditions, they may request your phone back if you break it now.

watch out cos if the guy spends a lot and he's still got your sim card, you're legally responsible to pay for the bill. You could always ring up and report it lost, or just tell them to cancel your contract, this might be a bit of a waste seeing as you can upgrade fairly soon (if you were thinking of upgrading that is) Also, try and see if the guy you sold it to is using your sim or someone elses.

What could go wrong would they could blacklist the phone and your credit rating so not only would you find it extrelmly difficiault to get a credit card/car credt/mortage/another mobile as well as the bloke coming back to you with a immobalised phone.

If you have filled your conctractual oligations to whoever sold you the phone (ie you dident have to stay on the tarrif for 4-6 months) then you can change down to a lower tarrif and just pay the line rental each month or as suggested buy out of it.

If you report it stolen the phone will be usuless the bloke will come looking for you and you will still have to pay the line rental each month.

And I rearlly hope you dident give him your contract sim with it 😉

No I just gave him the phone, I have checked out my phones contract, and from what I can gather the contract is on the sim and not on the phone.

I have decided I shall keep the contract going untill I upgrade then I shall pay the contract off, just so it is done and dusted.

Thank you for your help.