I picked up a single crank pulley to replace the double pulley. It slips over the crank but the crank key is too slightly too high for the slot in the single pulley making it a very tight fit (nylon hammer gets it on a few mm then I removed it)   
Has anyone else encountered this before and what was the solution? I'd rather do some research before doing anything drastic like filing the key or pulley. Thanks

Should be easy.
Remove the key by tightly gripping each side of the key with vice grips.
Clean out the keyway.
Check for metal burrs where the keyway slot meets the outside diameter of the crank. Lightly file if needed.  Check also key itself. Clean again.
Check internal diameter of the pully for burrs or damage.
Dummy fit pully with a light film of oil on both surfaces to assist sliding.
It ok refit key making sure the outside flat of key is exactly parallel to the crank shaft and key is fully home.  
If pully does not slide on neatly (only a light tap should be necessary) recheck for burrs or damage and that the key is parallel.
If still not working let us know.
 
 

Thanks Peter. I didn't know the key could be removed. I'll check it tomo Thanks

Pretty much what Peter said.
Parallel sides should be a nice neat fit, or else it will start to flog out the keyways.
Basically, remove key, clean keyways out, and give the key a hit with a wire wheel, or a light file to remove any burs.
It doesn't matter too much about the height if you need to file some off there as its the sides of the key which is important.
I deal with this pretty much every day at work.