Hi yawningdog, the above are not quite how I'd phrase the solution so here goes.
First download the rom update utility (ruu)from the link everyone has mentioned onto your pc.
Open the utility but don't tick the I agree box yet.
Holding the power button down on the Media Link itself (not the phone as it suggests) and connect it via usb to your pc .You should see the light flash red & green before letting go of the button.
My pc gave me a searching for drivers/device is now ready to use message.
Then tick 'I agree' in the ruu and proceed as with any other install. It takes a few moments detecting current version and then asks if you want to update.
Click yes and I think it says it'll be 10mins but it was more like 5 for me.
My results, my Desire HD videos now play instead of giving the 'cannot read MIME type' error. A vid shot on an I-phone no longer plays, no loss for me.2 DHD vids don't work(audio only) but that may have more to do with a recent unexpected reboot the day of filming.Buffering on all content is 2-3 secs.
Haven't tried anything else yet, will update if needed.
Hope that helps