I've done Sandhurst twice, although different course each time, and it was torrential rain both years.
In 2006 I had a stinker, coming way down the field and having to out sprint a club mate about 30years older who I usually beat by 5-6mins in the last 200metres!
Last year was probably the heaviest rain I've ever been out in, for 10mins before the start with no where to shield from it. A great day though.
I'm looking forward to Tadley, now they run the course the opposite way round, i find I can normally gain a stack of positions on those muddy hills bits in the last mile or 2. People who've been running like demons before suddenly find their pace reduced to that of a crushed snail 