I think we might have progress.
Well maybe not progress as such, as we are still having 'accidents', but I think I might have a better idea as to why this is happening. I don't really think it is a jealousy thing, so I've been trying to think of other reasons.
Thanks achan, what you said about B remembering me collapsing has stricken a chord.
Yesterday B was reading an Anpanman book which has an ambulance and fire engine in it, and he wanted me to read it to him over and over again. Then today he went straight for the same book again and I casually asked, "Hey B, do you remember when Mummy went in an ambulance?". He replied "覚えてるよ!" so I went a bit further to try and see if he really did remember and asked him what he was doing when Mummy went in the ambulance and to hospital. He then said "Poppyと一緒に遊んでた" (I was playing with Poppy). Which he was of course. If you remember it happened when my parents were here, and that's exactly what happened.
So this afternoon when A went down for her nap I filled up the bath (unusual for us to get in so early in the day) and we got in together and after a while he said out of the blue (quite happily) "赤ちゃんが来たね!赤ちゃんに待ってるよと言ったよ。". Which means something like 'the baby is here now aren't they! I told them I'm waiting for them.' So I pushed it a bit more and asked 'If you told them you're waiting for them why don't you want them to come?'. His reply?
"Mummy が病気になるから".
He said a few more things that lead me to believe he actually remembers me throwing up when pregnant with A, along with the ambulance trip.
Ok, I think I've hit the jackpot, don't you?
Now I just have to figure out what to do about it...
And just as an aside, no matter how much I speak to B in English he always replies in Japanese, hence the weird conversations above.lol