Thanks for the replies -
Usually there isn't faint whine that I can hear (not to say there isn't one, i've just never noticed it).
Now though, the (rather loud) whine is either constantly there, whether the head unit is on or off (as can be seen from the video), or,
the whine is absent, whether the head unit is on or off (at which point no sound comes through the speakers anyway, whether the HU is on or not)
This is leaning me towards the amp then, as Mike says it is always on - so if it's playing up the noise will either be there or not - regardless of the HU being on or off...
So...
1) Just to clarify, are we talking about the amp in the boot or behind the driver's seat
2) In the meantime, is there a fuse I can pull to kill it until the weekend??
Many thanks,
David