starter motor stuck, 1983 4.2 engine

[singlecoil]

It's not mine, but it has become my responsibility and I need to get it started, it's a DS420 limousine that's been standing in a barn for the last 7 years.
I've charged the battery and connected another battery across it, but on turning the key there's a click and a humming noise but no engine movement. Naturally I release the key immediately.
Being as it's an automatic there doesn't seem to be a way of dislodging the pinion (if that's what the problem is) so am I going to need to unbolt the starter motor? If so should I do that from underneath or above?
TIA

[Trooper2]

Have you tried turning the engine over by hand yet? If not, I would recommend it. Also, if you haven't already, try tapping firmly on the side of the starter motor while someone cranks the engine.

[singlecoil]

I have tried getting into a situation where I can try turning the engine by hand. I removed all the spark plugs and have found a 1 and 5/8ths socket for the crankshaft nut but the radiator and shroud is preventing me from getting a ratchet onto it. Turning the engine anciallaries just means the belts slip.
I will try the trick with hitting the starter motor as someone else turns the key, that might dislodge it.
If that doesn't work then I think it's going to come down to which piece of dismantling do I want to do, inlet manifold if I go in from above, ????? if I try from underneath, or radiator etc if I try to get onto the crankshaft pulley.