There are known issues with the 1-2 synchros on the M66W boxes and Ford did replace a few under warranty. As far as I know they are
not serviceable at all unlike the MMT6 fitted to other 6-speed models.
There's actually an M66W with a cracked rear housing on
E-bay at the moment cheap and if you were comfortable changing over the housing it might be a better option.
You could also try changing the oil - Ford recommend Trans Fluid BO-VC specification WSS-M2C200-C3 but some of the Euro guys have used a full synth equivalent with success.