Progress report 2:
Success!
Thank you to Ray
Took a dark frame and it appears the issue has been resolved. Managed 18 x 5 min frames in sequence before the clouds rolled in last night.
The graph spiked after each dither, but they eventually returned to the axis and imaging resumed. Curiously, the longer the sequence went on the quicker imaging resumed after each dither.
I'll post some graphs up tonight.
Cheers
AC