Hi Martin,
It certainly seems a bit disappointing regarding the faint outer extensions given the time you've put in, although there's still some nice detail in the brighter parts despite the seeing.
I'm wondering if longer subs would help? Perhaps the Helix is a dark-sky object only if you're hoping to go really deep.
I had plans to set up the larger scope for a few days given the clear weather we've had, but with the wind spoiling things I decided to just use the small refractor (apart from one night where the wind died down).