Thanks for the replies everyone. I've not wanted to give too much detail because obviously I'm posting about other people and it seems a bit unfair. This is probably going to seem like massive drip feeding now so I apologise.
Before this, I would say I trusted him 100%, something which surprised me as I never thought I'd trust a man again after what my STBXH did (literally dozens of affairs and one night stands and those were just the ones I found out about). I didn't want to press the issue too much yesterday because he was very upset at what had happened, but the truth is, this has introduced an element of doubt. This is what I'm most upset about as I promised myself that I would never stay in a relationship where I couldn't trust the person. I know from experience how much I would end up torturing myself. Now I'm in this position, I'm torn between thinking no sane person would be so nasty and thinking that I don't want to throw away a very happy relationship based on hearsay. This just feels a little too close to home for my liking.
I have posted about his ex before. When we first met, he was constantly cancelling our plans because his ex would click her fingers and swap and change their contact arrangements, usually so she could go out. He seemed to have trouble saying no to her. This changed when we started to discuss the future and the possibility of having DC together and I was honest and told him I couldn't see myself doing that given the current fluid arrangements with his ex. At that point he became more firm about them sticking to their agreed contact but I don't think she liked it much.
It's hard sometimes because being a SP myself, I often find myself sympathising with her position. In these instances, I will tell my bf my take on it and to his credit, he will usually listen. She has made some IMO, odd demands of me which after consideration, I said no to.
This latest argument, I think it's six of one, half a dozen of the other. The ex basically said she was feeling very stressed and asked BF to have the DC at very short notice. Genuinely, it would have been very difficult for him to accommodate this but he did offer two alternatives to her suggestion which according to him, she didn't like. It was then that she made the threats. She hasn't actually tried to contact me afaik.
BF seemed very upset about what had gone on. Up until now, he's never said a bad word about her so in my mind, this seems out of character but then I don't really know her, just the small snippets of info that BF has told me including why they separated etc.
When we talked about what had gone on, he used the line 'I wouldn't have told you if it was true' and 'why would would I ruin what we have together' but this sounded like the classic cheaters script to me (of which I'm well versed) and made me twitch even more. He also revealed that she made a similar threat when he was with his last GF before me which I hadn't known.
I just don't know what to think. I don't want to be taken for a mug but if indeed she has just said it to get back at him, I can't believe anyone would be that nasty, even to someone they're cross with and I'm upset that she's apparently thrown this spanner in the works. It feels like a lot of drama and I don't know if I can be bothered with it. I do love him very much though, and apart from this, he been a very good BF and seems like an honest genuine man.