Just got back there as far as I can tell. I'm not completely sure about this as I need to repeat it a good few more times before I can verify properly, but anyway, incase it helps anyone else :
Anapanasati seems to work very well with the first two jhanas, and I'd guess the second two as well by the looks of it. I sat for around 15 mins and felt the first one arrive I think, then the unexpected happened and I might've crossed into the second a few minutes later. One thing was key with this though, and that was that I was firstly on the 5th step of anapanasati, sensitive to rapture, then when I decided to move to the next step, sensitive to bliss, it might have switched me into the second.
It mainly happened because this time I didn't get anxious or involve the "self" or "I" in the whole process (or at least it was minimized a great deal.) I wasn't in a brilliant mood when I sat down so couldn't be doing with the games the self plays to try and distract from concentrating in meditation, (getting excited/anxious about jhana which allows hindrances in, craving jhana, getting caught up in hindrances for other reasons, thinking about various other things instead of concentrating, and so on.) but this unwillingness to get involved with all of that continued as the jhana arrived, so maybe that was the main factor that allowed things to move further.
Another big thing was to refrain from getting caught up in the feeling of the jhana itself, and to remember that the main aim was to meditate for insight. That unattachment also seems like it helps with allowing the jhana stages to flow more freely.
As a last thing, it looks like the 5th and 6th steps of anapanasati work greatly for entering the first jhana, then as you continue through the feelings tetrad you can eventually work your way up to the third jhana as you work on the 8th step : calming the feelings and perceptions. It seems to work perfectly with allowing the pleasant/blissful feelings to start to drop off as you head towards equanimity, (at this stage I purposefully came out of meditation though. The main reason being that I've read about the problem of trying to attain various stages of jhana too quickly, in the sense that you should familiarize yourself with each step first before moving on, or you risk not being able to get back to the lower stages and mess everything up.) then it looks like you maybe come to a junction : you can either go for the arupajhanas and stop anapanasati (in the sense of stopping going through the 16 sutra steps, not stopping being mindful of the breathing.), or you can stick with anapanasati after hitting the 4th jhana and then go towards the insight meditations in the final tetrad. Or maybe when you're in the 12th anapanasati stage of releasing/liberating the mind you can work on infinitudes of space, consciousness, etc, there too.
Anyway, some of that is clearly going to take a fair bit of meditation to suss out, but thanks either way Thomas for the tips, and hope your meditation is going well too.