Thank you all for your encouragement and very kind feedback.
Paul - 111hours for Helix is a very impressive and inspiring effort!
I'm up to about 38hours of Ha now and st.dev. in the background is around 1.3ADUs. OIII was collected on darker nights so 30hours also gives around 1.3ADUs of st.dev. for the background.
The faintest bits that I can confidently see get on average around 1 photon per pixel per hour of exposure. Subs are 15-minute long and I think it would have been better to try somehow longer subs. But random quickly passing clouds are not uncommon over here, so 15-minutes helps a bit in minimising the number of subs that need to get rejected.
Moon is bright again and getting close to the Helix, so I may need to wait a bit to finish collecting data.
Instead of trying to go deeper, I'm thinking of trying a third filter...