I don't think that you have any obligation to inform the HSE. As far as I'm aware, it's the employers responsability. If it happened on another site, with a piece of kit which is't owned by your company then I'd say the chances of them getting hit with any fines are minimal. They provided you with the relevant PPE which it sounds like you were wearing. If on the other hand, the kit that hit you is owned by your company (think a hinge failed if I remeber right) then so long as they have maintenance and inspection records which are up to date for that piece of kit and the accident couldn't have been foreseen, then they should also be alright. Against common myth, the HSE don't try to crew you at every turn (although the inspector will have an important part to paly for this).
Under RIDDOR (Reporting of Injuries, Diseases and Dangerous Occurences Regs) yor injusry is classed as either a Major Injury or an "Over three day lost time injury". Either way it is reportable to the HSE within ten days on the F2508 form. Failure to do tihs is in breech of the RIDDOR Regs which forms part of the Health and Safety at Work Act. The rules are all open to interpritation , but that is what is laid out in the HSE Guidence. Have a look onthe HSE website under RIDDOR and there should be some stuf there. I should imagine that the major argument for your company would be that it was on someone elses site but without knowing all the details, if I was your company I would have filed the report as well as it is one of my employees that was injured. I knew that course would have come in handy one day :D
Glad to hear things are getting better mate. If they payout isn't very much then you culd always send the video to You've Been Framed :D
I remember when I was off my feet for months, I was so glad to get out into the garden when it finally happened that I even used to go outside in the rain just to remember what it felt like.