Thank you for all your replies - some very useful advice for dealing with my parents in general!
In answer to a few of your questions - yes they were getting suspicious that 'something' was going on - the kids were mentioning my boyfriend's name a fair bit. I thought it was better to give them the facts than have them invent things in their heads, and at the time we had the conversation, it went very well, a few minor objections, but generally no big reaction. I constantly live in hope that my parents have changed but I was half expecting some kind of backlash.
I'm not really worried about any custody issues - they are constantly criticising how we look after the kids, despite the fact that they are all well behaved, well adjusted and happy, but I don't think they would ever go that far. We would also have the support of my husband's parents if this happened. I suspect that this conversation, if it takes place, will be mainly aimed at making us feel guilty, I don't think they will actually have any action in mind to come out of it.
In general, if I was giving advice to anyone else who wanted to come out to their parents, I would suggest giving the minimal amount of information necessary and not think that the conversation's going well, and hence spill all of it.