Don't know what's causing the following but any help much appreciated.
Did the 85k Selkirk last weekend on the HT and on the final couple of descents (fast and bumpy) I heard a noise similar to a squealing brake disc but on looking down my chain was slack (I actually thought it had snapped). It took a few pedal revolutions to get the chain taught again, but it happened maybe 10-15times in 10mins of riding. I replaced the freehub body (Mavic Crossmax) about a month ago & cassette/chain 3 months old. I can't recreate the problem on the road so I've no idea what the problem is.

Sticking jockey wheel?
Freehub rebuilt with grease instead of oil?

Jockey wheels checked and both running freely. Freehub was replaced by very experienced mechanic so guessing that's fine too?!

I'd blast some gt85 or similar down the back of the cassette, maybe it was just a piece of mud or similar getting stuck and catching things.

It's probably a worn hub body. If the freehub was run without oil for any time the bush would have worn down the face it runs on. There's a guy in the states that sells oversized bushes to cure this. Not sure of his details though.

It's usually the bushing on the inside of the freehub body running dry and oscillating around the pawl seat body on the hub shell. Even though you've replaced the freehub recently, it may still be the cause of the problem. It's common on Crossmax wheelsets. You might have had a M40592 (m10) body supplied instead of the correct M40580 Xmax version. The beraring spacing is slightly different on these 2 bodies, and the Xmax has 1 line around the body, where the M10 has 2 for I.D. purposes. The Xmax body sits further inboard onto the pawl seat and butts up against the labyrinth seal, and the M10 will sit about 1mm outboard which puts the 11t sprocket really close to the frame. It just doesn't have as enough support to work properly on the Xmax hub as it's designed for the Ksyrium and Xtrail/ Xride style wheelset with steel axles fitted.
If it is the correct body, it needs lube to cure it. A good quality freehub grease or light oil will sort it.