I have recentley change the ball cock on my hot water tank and in doing some small peaces of chipboard went into the tank and into the pipes now the water flow is very low how can I resolve this

We are speaking about a storage tank and not a water heater, correct? About the only thing you can do is to backflush the pipes in question. Good luck, Tom

How do you backflush water pipes

Perhaps I misunderstood your question. I assumed it was the drainage that was slow.
Please give me more information. What pipes has the slow water flow? Describe the set up that you have. Are you in the UK? Is this a hot water storage tank or is it some sort of a water heater? You back flush a pipe by opening the terminal end and applying pressure to blow the debris back out. Is it the ballcock that has the weak flow or is it the gravity drain from the tank? Let's get specific here. Regards, Tom