RichPurn, I don't know if you're a cager also, but I'm sure you're aware that all cars have "blind spots" from their mirrors, as do bikers. Basically, if a biker/car is anywhere alongside and/or slightly to the rear of a vehicle, there is every chance he can't see you in the lane next to him.
You said you were in the right-hand lane. If you think about it, that becomes a much larger blind spot area for the driver and extends even further back. If you had been in the middle lane (assuming three lanes), he'd have had a better chance of seeing you coming up behind.
If you were "gradually" overtaking in that right-hand lane, you could have been in his blind spot for a very long time before he moved over.
I don't know, I wasn't there but maybe that's what happened. I've seen many bikers putting themselves into that vulnerable position - in traffic, as well as motorways. Let's hope your post will make a few more aware of the danger.
Horn on a motorway? Well, I have fitted a powerful air-horn but still doubt it's efficacy in that scenario.
The moral I'm getting at is this: Position your bike to be seen by others at all times. Don't sit in their blind spots.
AND... It's not what you see, it's what you CAN'T see that'll get you!