Great replies.
I think I'll do the bike safe course.
My motorway filtering is usually up to around 60mph, depending on density of traffic, lane width, presence of junctions, bends, lorries etc. Also I'm less inclined to filter in the dark or in the wet. (On normal roads I don't break the solid white line or overtake on zigzags.) I adjust my speed depending on the traffic around me, maybe only 10mph faster.
Never had a sketchy moment due to speed, but touched the curtain side of a lorry with the handlebar at about 15mph. (No biggie).
Most drivers are pretty good about it, particularly in slower moving traffic. I think I'll probably need to calm down the faster stuff. Not because I feel unsafe (I don't) but it can seem a bit anti-social and might not be looked upon too kindly by the powers that be.