Unfortunately not everyone passes their test first time and waiting times for another test can be frustrating. The least amount of time a candidate must legally wait for a retest is 10 working days so that there is sufficient time to retrain and correct the mistakes made on the test.  But rarely can a test be found in this time and lead times can be as long as 9 to 10 weeks.  Waiting for cancellations can mean sitting on a computer hoping to catch a time when a cancelled slot becomes available and is obviously time consuming. 
 
The following link has been approved by previous candidates as a company that will take do the work for you and look for cancellations on your behalf.  It is not a link officially approved by this school but has been proved to be a useful tool.  As this costs money the school does not take any responsibility if the results are not to your requirement.