Thanks for your replies guys, I really appreciate all comments, experience and advice.
I'm not offended by any comments, they need to be direct to aid understanding I guess.
I've not been to Al Anon but have had some counselling, although not too recently. I need to get myself back on track after this current episode.
We are very lucky to have her parents live five minutes away and they had to cope with her binges for 10 years, so they understand pretty much what goes on and are always at home so in real urgent situations, especially if I'm not close by, they can step in pretty quick if things turn catastrophic, which they haven't ever done to date. My GF has so far been pretty conscious of what our son needs when she's on her own with him and manages to contain herself until I'm back from work, although she doesn't always completely abstain unless I'm not staying away which is 95% of the time.
The local social services are aware of our situation since I contacted them last year about it and although they aren't getting involved yet I will not hesitate to get in touch if there is ever a serious risk to our child. They are pretty quick to act when a small child is involved. Once these guys or the police starting to get involved things may deteriorate so it's not something I really want to do keenly unless it must happen. It's just that I don't know if we're at that stage right now and I need to separate for our son's sake.