My non-o visa extension is due to expire in the middle of November but I expect to be out of country working for about 4-5 weeks from end of october/beginning of November into December.

Pattaya immigration have told me that I can only apply for it within a week to 10 days of the expiry date. I explained to them that I'll be outside and they've basically told me it means I will lose my original visa and will have to apply for a new one overseas - at considerable more expense.

I told them I would gladly apply for the extension in September or October and loose a month or two of my extension time on the existing extension but I've been informed by the immigration officer that it's not possible to apply outside the 2 week expiry window. Anyone working rotational work and living with family in Thailand with a non-o visa may have had similar issues. Would appreciate any info from anyone.

Who at Pattaya immigration told you that?

You can do it up to 45 days early. This has been confirmed by reports on this forum.