OK so I've just bought a Genustech variable ND to use with my 24-105mm EF lens - problem is I can't put it on or take it off with the lens hood attached, and if I take the hood off to put the filter on then the lens hood will not fit over the filter!

Anyone else come across this and come up with a workaround?

Can't use hood with ND Filter. Beside, you would not want a hood any way because you will need to rotate the ring to get desired light.

Eclipse ND Fader 77mm Variable ND Filter - Genustech (http://www.genustech.tv/products/eclipse-nd-fader-77mm)

Theres an optional hood available or maybe use any cheap 82 mm hood (might work)

I have a cheap, collapsible rubber lens hood on top of my ND filter like this:
$5 Rubber lens hoods/sunshades - Collapsible with 3 stages. - YouTube