Clutch dragging - Steve Greenald Mapping Session - R500D/Quaife sequential

[Shortshift]

Coming to the end of a mapping/rolling road session yesterday (Steve Greenald working his usual magic) and time for some power runs.  Up she goes, max rpm, onto the run-down phase and then - a whiff of clutch and looks of consternation. 

With a sequential, you can't just knock it into neutral for the coast-down section so the trick is to keep the clutch fully depressed till the run-down is completed.  But with clutch pedal fully depressed it seemed as though the clutch was gradually re-engaging, not holding in the fully depressed position so beginning to drag slightly, which is not a nice situation when you think about what's going on with the rollers.  Anyway, brought to a halt without any ill effects but we couldn't risk doing any more power runs till the problem is fixed.

Ideas?  I'm not aware of master cylinder or, fingers crossed, slave cylinder issues (it's a concentric jobbie in the Duratec, so an engine out job to replace) and the only change I've made recently is to replace the DOT4 fluid with DOT5.1.  Temps were quite high, of course, with a lot of heat soak into the block, bell housing and transmission casing from several hours of roller running, but I can't see that being a significant factor.  Only 7,000 miles, never raced or rallied - you get the story.

Anyone had a similar experience, or got any ideas?

James