Although MaximDL and SGP both control the Nightscape through the ASCOM driver they appear to do so in a different way. I suspect Maxim comes with the driver pre-bundled.
For SGP, I downloaded the ASCOM driver from the Celestron Web-Site. This version of the driver points to the AstroFX folder even though I not installed AstroFX on the PC. The only solution was to also install AstroFX from Celestron.
Finally, I had to make sure that the NightscapeASCOM.dll file and BCGCB632.dll files were within the AscomFX folder. Once this was done, SGP could locate the Nightscape driver and all works fine.
I am sure that there must be a neater way to achieve the same result but this worked.