The preferred way to file a flight plan is online, and a website giving full instructions on how to do this is here.
You may need to set up an online account, which you can do here.
A copy of the flight plan form is here.
Should the worst happen here is a form to report an incident to the CAA.
Apply for Hangerage at KHA
CAA Re-validation Form : There is now an on-line option to submit revalidation information