How dry does it have to be to do the main hang? is there a rigging guide on t'internet somewhere?

I assume you mean the main shaft and not Gib Tunnel. I have never seen a rigging guide but its quite straightforward. Drop off the end of the ledge and descend the corner until you can enter the "A" rift with no floor. Traverse through the rift with ample bolts to the Y hang at the end. Very airy! From here the rope drops right down to the floor of the chamber just between the two falls. One off Birkbecks ledge and the other from the Spout.
When I last did it the descent was fairly dry on the way down. However it rained while we were down and I got a severe battering on the way up. I would avoid it if any chance of rain.
Dihedral is ok in moderately wet conditions if a little windy and spray lashed. But I would avoid even this in severe flood.

Rigging topo in Elliot and Lawson's SRT rigging guide - now out of print.
For the direct route there used to be a guide line to pull yourself into the roof rift, very airey!

It has been pissing down, by the way, in the general 'western dales area'. If you have already done it, I'd be interested to hear just how wet it was!

Thanks for the info, its my 21'st on Sunday and I was hoping to rig one of the GG big 3 routes to clebrate (dihedral, main hang, rat-hole) looks like it will have to be dihedral (especially as I have a permit). Ho wet is too wet for dihedral then? Is it looking likely that dihedral will be doable?

Definetly stay out if the river is flowing well! The spray lower down goes every which way and is very disorientating, and that was a dry-ish day, reckon it would be nasty in the wet...
Have a look see, be prepared to go elsewhere, judge the weather and have fun

4 initial bolts lead to a deviation (in place) at -1.5m and a 10m abseil to the start of the rift. The rift itself has 5 bolts and is very airy. You need 30m of rope to get to this point. The hang below is 85m according to Elliot and Lawson (but I haven't actually measured it to confirm).