Morning all, get a 
here's your update:
DH and I talked for hours last night and I feel we have got to a very positive place.
It turns out DH is not and has not purchased any DVDs, rather he enjoys looking at the DVD covers, particularly the back covers where there are screenshots from the film. He has shown me his email account and his bank account and there are no emails or transactions that aren't legit, when I asked to see them he gave me the passwords with no hesitation whatsoever. He was using the shopping basket as a way of creating a 'playlist' of DVD cover images that he liked to go back to.
I asked him if I would find any DVDs or mags in the house and he straightaway admitted to the ones I had found that day, no hesitation.
we talked about the incest nature of the DVDs he had selected for his 'playlist' and he says it had never even crossed his mind, he wasn't looking at the words or the story (and as he isn't watching them, just looking at the pictures he's not watching the storyline play out) its just that as he was clicking around looking for images these ones jumped out at him as they show a slightly older, more 'normal' looking man rather than a porn star man, with a younger, more 'normal' looking worman, rather than a highly made-up porn star type woman.
he can completely understand why having looked at the DVDs selected I would be appalled and sickened and can see how I have made the connection about DD and the thought horrifies him.
Once or twice in the conversation I thought he was saying what he hoped would get the 'right' answer from me - so when I asked how he would feel about me looking at mother/son porn he started out with 'I would try and understand', I called him out on that and then he admitted he wouldn't cope well with that and that yes, it would make him question me and why I would want to look at such a thing.
I was happy with the answers he gave, I recognised a few attempts at lessening the impact and called him on it and he then gave up the truth and I do believe him. you may think that is naïve of me but I know my DH, he is not that good an actor. plus I have analysed his behaviour towards me and the DC, thought back over his behaviour during sex and there is absolutely nothing that would in any way give me cause for concern.
we moved on to what had got him to this point. He explained that he had felt like I only wanted him as a sperm bank - everything was fine, then I fell PG with DC1 and was so sick and uncomfy that sex was off the table. then you are in to the natural slow progress of getting your sex life back after the arrival of a DC, and I had a bad tear so it was very slow progress. then everything rams up to multiple times a week to conceive DC2 and then back to nothing when I get PG and sick, and repeat for DC3. He wasn't blaming me at all, just explaining how hard it has been for him to deal with the apparent hot and cold nature of our sex life. which looking from his perspective I completely understand. He also explained how he feels I brush him off any time he attempts intimacy, even just a hug.
So then I got a chance to explain my side, that I am at home with 3DC all day who use me as a human climbing frame, I feel I am being mauled all day long so the last thing I want in the evening is someone else mauling at me. I also explained how I know we don't have as much sex as we would like to have and because of that I feel reluctant/nervous of accepting a hug or more than a chaste kiss incase he assumes sex is on and I'm too tired for that. I explained that sometimes I just want hugs without feeling obligated to go further, that I need more base level affection to have a chance of being in the mood.
DH was clear that the porn use is only because our sex life as become non-existent, that he doesn't use it when we have had spells of good, regular sex. He is using it sometimes a couple of times a week, but sometimes goes a couple of weeks without. (this is backed up by how often I have seen the websites pop up in the 'new tab' page.) He says when our sex life is regular (1-2 a week is what he would like) he has no need and feels no urges to look at porn.
without boring you too much more, the outcome of everything is that he will not use the porn going forward. We are going back to the beginning and starting with holding hands, kisses and cuddles. I have said to him he is not to try and initiate sex but focus on us having lots of base level intimacy - hugs, kisses, just every day body contact. I have said to him when I feel ready I will move us forward to having sex but that he has to go at my pace. he has agreed readily to this saying all he really wants is for us to get back to a normal sex life. which as we talked I realised is what I want too.
we have also agreed that we will talk more openly about sex going forward rather than getting frustrated and in his case turning to porn, and in my case just turning off altogether!
I feel very very positive going forward, I feel my faith in my husband is being slowly restored and I hope we will get back to a happy, healthy sex life in the near future.
thank you to everyone who offered advice, especially those who took the time to PM me and share their thoughts and personal experiences with me. You have all (well nearly all) been very helpful in giving me the courage to have the conversation that needed to be had. so thank you 
heading out for the day but will check in for your thoughts later!!