Ongoing Web Design Contracts?

I'm struggling with my web design contract. I've got a contract for a fixed project, useful for new builds, but have a client who I do pieces of work for. So I've recently done £100, then £120, now £70. A contract with a completion date doesn't seem relevant. Does anyone have examples of contracts for ongoing work?

Here is my current contract: http://pastebin.com/9CpDhQq1

Do you know is it ok to just write anything in the contract? It seems like on one hand it needs to be informed by proper legal advice and on the other you can write anything and as long as they sign it it's legally binding. I don't want to pay a solicitor until I know if I want to do this work full time.