It did occur to me that I had to ride a bit harder to clear the gunk out of my tyres and get the necessary momentum, but each time I tried accelerating a bit I just stacked, twice spectacularly – there was blood!
The Col de Coux ridge descent on Saturday (closer to the wet day) was fine – I have some class photos of that descent in zero visibility conditions which I'll put on here when I get time. Had to walk a few bits but generally OK.
The Le Bourgeoisie descent into Samoens on Sunday morning was where I began to have problems.
I then did a descent in a nearby valley down to the Giffre river (big thanks to freeridenick for some really class tips here) which in the dry would've been one of the best descents I've encountered in the Alps, and it wasn't even that wet, but the serious mud at the top clogged my tyres turning them into slicks and the trail lower down was just too technical and just a little too slippy (for me) to get the necessary speed up to declog them.
Thanks for the tips on tyres.
nb none of this is to suggest I didn't enjoy it, any day riding in the Alps is a good day!