Duncan,
I’ve had Contractors do mole draining on a number of sites in the past - esp. a really shitty, boggy upland marsh which some eedjit thought would be a great site for a footie pitch!
It really is a great (Cheap!) solution for moving water away very quickly. Obviously, you still need somewhere for it to go!
I’ve tried both back-filling the slit with sand and just leaving it open. Both seemed to work, maybe the sand-slitting a tad better, but very much site specific.
Don’t bother with pipes. That rather defeats the idea and you’d never be able to pull a perf pipe through anyway. I’ve seen solid pipes pulled but they were for a water supply.
Cheers,
M.
PS Agree with what Jon says re tractor. Low ground pressure tyres sometimes struggle with traction and normal tractor tyres will tear up your surface. Trial in non-crucial areas!