I need to withdraw the comment below having now successfully brought the guider to focus in an SBIG STXL FW8G filter wheel. The issue has always been down to the fact that the offset value for the KAI340 chip was incorrectly set. So while I thought I was getting image data, albeit intermittently, I wasnt and this drew my conclusion that the guider would not come to focus on an f3.8 system.
The offset problem was solved today by DL,and the guider was brought to a successful focus (right at the end of travel of the adjustable guider).
That's good to know, I have considered moving from the QHY 7x36mm to the SBIG 8x36mm for that extra slot; a 3nm SII would be nice or maybe even a IR or 200 spectral grating. Damn! Now I need a 10x36mm CFW