Will I be able to keep my contract?

I'm on a talk mobile contract now and have a sony phone. If I went to buy a iPhone of eBay would I still be able to keep my contract and would talk mobile give me a smaller sim for the iPhone?

Your asking the wrong people. Go into a local talk mobile store, and they'll be able to tell you everything you need to know, if thats not convenient, click the link below for more accessible options.

http://www.talkmobile.co.uk/help-and-advice/contact-us

Your phone service provider does not care what phone you are using, but you may need to get a new sim card if it is a different size or capability. Larger sim cards can be cut down to size also.