The money claim online is great.
But, it does stipulate that you should try every other reasonable means to recover your money first.
So if I were you, I would put the cheque into the bank and see if it clears first.
Then, if it doesn't, your bank should return it to you. Contact the client and politely explain that the signature on the cheque constitutes a written acceptance of satisfaction with the work you carried out. Explain that you are prepared to take legal action and that he will become liable for the costs. You must give him a reasonable amount of time to send another payment - usually 14 days.
Try to do everything in writing and keep copies. After doing this, if he still refuses to pay, then fill out the claim online. He will receive notification from the court in a matter of days and will probably call you and be a bit upset.
Explain to him that you can withdraw the claim at any time, but there is a time limit, where the court will pass judgement regardless. Also explain that it is unfortunate that he must also now pay the £30 cost as well as the money he owes you before you will withdraw the claim. And just to ensure things run smoothly, tell him you must receive a cheque at least 5 days before the deadline to allow it to clear.
Most reasonable people would see you are serious and pay up straight away. If he doesn't, just leave it to the court.
If, and when, he does pay you in full, it is a simple call to the court to let them know you are withdrawing the claim.
Hope this helps...