As Will says, whack spanner really, really firmly with something very heavy (I prefer a big chunk of wood, or a really big carpenter's mallet, to a hammer - less easy to miss the mark!).
I find that the inertia of the flywheels is usually enough to get things moving.
And I also reckon you've got to take the engine out to get the gearbox out completely - I gave up (after much trial and error) on getting the gearbox out without dismantling the engine when I replaced the oil seal on the box in my '56 VH.
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)