Hi, I'm quite overwhelmed with the amount of replies. Thank you for all the advice! I tried to reply on my phone but knew I'd miss something. So I thought I'd wait til I got back home to the laptop. Sorry for long incoming post. I don't want to leave anything unanswered.
When we looked into it, we found it would cost at least £150 to get the test done, but maybe a bit more. I think it depends on how many samples you're testing. It would need to be at least 3 - me, DP, DS. But we would want to find a very trustworthy, regularly reviewed test, as it's such a sensitive thing.
At first we decided we would take the test and then contact them with it straight away so they would back off. Then we decided we would take it and have it stored ready to give them if they ever ask again. More recently, we've not known what to do. Now, DP doesn't want to do the test at all. I only want to do it just so that I know "it's done" and I don't have to worry about it anymore. DP has insisted the entire time that he knows from the emails and dates that he's definitely DS's dad.
There are no nagging doubts, at least not rational ones. At most, I have thought "it's only possible if 12 month pregnancies are possible". You hear about all sorts of weird medical things happening so I googled it, and they aren't, so that doubt is gone, unless I'm feeling particularly panicky and the thought reoccurs, which is obviously the case with anxiety disorders, that the anxieties are very rarely rational and return with a vengeance whenever I'm feeling edgy or vulnerable.
There's no doubt that the dates don't add up the way the ex wants them to. Essentially, in the October, ex found out DP and I had moved in together (it all happened very quickly) and sent DP an email to tell him about the one night stand, with a "You deserve to know what she's really like" kind of tone. The email says "a month ago", which would suggest September at the earliest, but then he also references my grandad's funeral as happening after the one night stand. Well obviously I know when my grandad died, it was the end of July. The funeral was about a week later, start of August. We've double checked this against things that were said on Facebook and emails at the time with other people.
DS was born the following August, 3 days overdue, normal sized baby. All the due date calculators say I had to have conceived mid-Nov. This date is confirmed in my first and second scans, and of course, the day he was born. November is also the month where we went on a bit of a boozy holiday and had a lot of sex.
If it wasn't for the emails, I might be able to be a bit confused about it all, as it was so long ago now (DS is almost 6) and a lot of big things were happening in my life at the time that make it all quite hazy.
6 years is a long time to have such a grudge for someone that he's still trying to play games. But I know it damaged his pride a lot that I refused to get back together after he ended the relationship and later regretted it. And I know that if he is playing the same kind of games with his wife that he used to play with me when we were together, that this is something I wouldn't put past him.
DS looks like DP and everyone comments on it. There's just no way at all. It's not possible.
I know something has to be done. If I didn't have the anxiety problems, maybe I could just ignore it. The reason I haven't been able to is because for my own piece of mind and being able to avoid that extra bit of panic, it felt better to resolve the situation in everyone's minds officially.
But I feel the same as most of the comments here, in that I shouldn't have to do it. I feel like it's unfair as even though it will get him out of our lives, it will also give him some smug satisfaction that he has managed to convince DP to not trust me (which isn't the case, but I can't exactly say as much without looking like someone protesting too much). It's our money and our emotional investment, while he just sits around with his new little family making them feel uncomfortable that he might have another child who will suddenly need his money and attention at some point.
I have never initiated contact with ex MIL. She has always been the one to contact me. Sometimes, I have been irritated, as it's seemed like she's messaging to show off about how well ex's life is going. Other times, it's like she needs to vent and knows I'm the only one who understands how hard ex is to deal with. She even said as much, that she doesn't know how to talk to his wife about "the way he is". I reply politely through the conversations but I find it a bit uncomfortable most of the time. I really say as little as possible.
I don't like confrontation though, so I usually get stuck in conversations with people. I'm going to work on that.
I know she can be just as manipulative as her son though. Once she backed up a lie I eventually discovered to be false about the ex, when he suddenly got a "potentially fatal illness" weeks after we first split, amongst other things. I've never confronted her about these things.
I think my current feeling is to change email addresses completely to my new one and stop even checking the old, block them all on facebook, even cut contact with the friends we shared when we were together. Luckily none have my new number or address. I just worry about my own state of mind and worrying about them finding some other way to contact me or whatever. I dwell and panic way too much.