Perhaps I am wrong but i thought I may be able to get away with inserting three lots of SI Drums so that i could pan and use different effects on certain ones without it interfering with the others. When i try this i can only get sound out of the first set of drums that I insert to my project. Is this a feature or is there something else i need to be doing? I appreciate that drums are 'usually' played on channel 10 and that this may be the problem but in the past I have got round this by assigning them to channels 9 and 11 as well on a Soundblaster card.

You can have as many instances as you want within the limits of MC6. Each instance of SI-Drums requires its own  instrument track or MIDI track and audio track. Since they are on separate tracks, the MIDI channel does not matter. Moreover they will not interact with each other.

there is no requirement to assign drums to channel 10 and is not even practiced any more. 
 
 

Thinking the same as scook. Make sure each instance of SID have their own midi track assigned to them.