Hi all. Thanks for your very kind replies - I've lived to tell the tale, and it's really good to hear from you all and know that I wasn't being unreasonable or over-reacting; sometimes if someone tells you that enough, it's hard to be sure of yourself any more :-/
You're probably going to think I'm a stereotypical case and in denial when I say that he honestly isn't like this and that he does love me and treat me well - sounds like the beaten wife, doesn't it? But the problem is this friend he went to see, as pathetic and deluded as that probably sounds. We've only ever had a big argument when he's been out drinking with her - there's something about her that seems to goad him into becoming a totally different person and one with very little respect for me and my feelings. We've talked about it a lot today because last night, like the last time he was out with her, she took it upon herself to get involved in what she thought was an 'argument' (at that point, nothing more than a minor disagreement that we were sorting calmly), and fuelled it into something else. She then took it upon herself to start texting me, claiming that DP had asked her to say this and that as his phone was dead (the phone dead bit was true). When I challenged him about this later, he had no idea what she'd been saying. I've told him that he needs to sort it out with her within a week and that if she can't understand and agree to keep out of it, he needs to choose whether he wants her or me because I am not prepared to live in fear of this happening again.
I've said similar to the friend as well. I think the main problem is that they've been friends for years and she seems to feel some kind of ownership over him, which isn't helped by the fact that she's also attracted to him. She's very patronising towards me, referring to me as 'pet', 'darling', 'sweetie', etc even though I'm slightly older than her and we don't know each other at all, and giving me crap like 'you've got to understand...' and 'DP feels that....' etc, and offering for him to stay at hers rather than come home 'if that would make me feel better'. I showed DP the thread on here and left him to read it. I think it finally got it into his head quite how bad he's been - it's never nice to see yourself condemned by strangers I suppose.
Re someone's suggestion, there's nothing going on between DP and his friend - though I can more than see where you're coming from there! Thankfully, apart from DP being someone who despises cheating in any form, his friend is the opposite of me in most ways and also very overweight and extremely slutty - two characteristics DP finds a major turn-off. I think I'm safe there!
Comments/feedback/suggestions/etc very much welcomed, and thanks once again so much for your support and indignation on my behalf last night. I'm really new to Mumsnet as you've probably gathered, and it means a huge amount to be accepted enough for you guys to care how I feel. Thankyou :-) x