There's generally a clause that you can terminate the contract by giving 21 days written notice. It should be a standard PSRA contract. But, if you sell to anyone that he introduced, then you still have to pay his commission.
Re selling yourself, some personal thoughts. I'm buying at the moment, and I've seen a few houses advertised privately. But I haven't gone to see any of them. The last thing I want to do is meet the seller before I've made a deal. I'd only call about one of them if it met all my criteria. That might be a personal thing.
I recently sold, with an agent. Went sale agreed quickly, which turned out to be the easy bit. There were a few times over the period to close that it could have fallen through if it hadn't been for intervention from our agent. They made the difficult phonecalls I wouldn't have been able to. I realise I got lucky with our agent - I've dealt with over 12 in the buying process, and have only met 1 who was doing any real work.