WEll just to update, though i havent' exactly solved the problem.
I pulled down the compressor (as i love taking things apart anyway) . The motor works great, as does the rotor for the compressor. At the top of the compressor, there is a disc with holes cut out that spins with the motor,(though it is geared down), and opens and closes the seperate air outlets for the horn.
there are four springs under the disc that pushes it up against the top plate where the air hoses attach. This is where i believe the problem is. the plate is pressed hard against the top causing enough friction to labor the motor. (with this plate removed, and only the rotary valve and motor in place, it spins like a champ)
Loosening the top plate will relieve the pressure and the motor will spin faster. the only problem is, too loose and it causes an air leak.
I think i might fiddle with the springs abit and see if i can loosen it up.
Voltage to the motor doesn't seem to be an issue. with the motor off, it's 12.8 volts, with it running (and when the glow plugs click off), it's 13.5.
btw, i lubed the compressor with some sae 30 oil (as it was handy). I'm not sure if this would have been the optimal choice.
edit: got a mate to record the horns on his phone. apart from the atrocious audio fidelity, you can make out how slow they are, but never the less some what cool
My horns (actually a quicktime vid)
http://mr-t.gotdns.com/maestro/windale/dixie.3gp
what they should sound like
http://mr-t.gotdns.com/maestro/windale/dixie_horn.wav