10 litre sac for me - just big enough to hold waterproof top and bottoms, light fleece, buffs and gloves, cavers survival bag, emergency rations and first aid kit.

I use a shoulder strap mounted 1/2 litre water bottle which is perfect.

The food and drink on course is so good and plentiful you really don't need to carry anything other than the emergency requirement.

Wish I was doing it again this year but just found out that I got into the Fred Whitton cycle sportive which is the day after so think I'll give it miss! (now there's the makings of an extreme triathlon.....)