Hi Lost - I'm a bit behind on your thread and trying to catch up on your most recent posts. I must admit that I was a bit concerned to read that you've come away from Relate feeling as thought you felt your marriage needed a big kick up the bum. You said "For the last couple of years we've grown apart a bit, but I admit I have been colder with him since I saw that photograph in his phone." I kind of get the feeling that you're trying to find a reason somewhere as to why he might behave this way - I totally agree with the other posters, particularly Mad and Five, who say that he didn't need a reason to do this.
You might have been colder towards him because you felt that he breached your trust and lied about that photo - that's not your fault. That's a natural reaction to that type of situation. If he was concerned about your reaction, he should have talked to you and come clean about the photo and its origins. If he feels that instead of talking to you, it was normal for him to pull away, buy a secret phone and start chatting to some ex-colleague on the sly then he's the one with the problem, not you. You've not contributed to this in any way - do you hear me? Not in the slightest. You must quash those feelings of guilt at your coldness as soon as they arise.
If it makes you feel any better - I did exactly the same 
For a good couple of years, I convinced myself that H turned to another woman for attention whilst I was pregnant because I became "obsessed" with being pregnant and then with my DC1. Guess who put that thought in my head? and he said exactly the same thing - that it wasn't my fault but it was just how he was feeling at the time. Except now, I can see that it's total and utter bollocks. I remember that time clearly - I was totally in love with DC1 from conception and yes, I did "neglect" him but he showed absolutely zero interest in me, the pregnancy or the baby. He never tried to get involved, he never spoke to me about it. Instead he absented himself by disappearing to his study every evening to send dirty texts to another woman and ring her the minute he left me to go to work. Did I reject him? Did I bollocks - I still cooked his dinner, looked after him and dealt with all his shit.
I'm betting that for the last 18 months, even if feel you've been "cold" towards him, matters have pretty continued as normal for him. I doubt he even noticed, except when he's looking for an excuse for being a total knobhead! I did have to LOL at the MN comment as well. I did exactly the same with H. I even left my thread open for him to look at - his reaction? Not what I was hoping for. Apparently Mumsnet has ruined his marriage. Um, no - that'll be you when you told another woman you loved them....
I'm very aware that I'm ranting but whilst I'm on a roll, what a line of bollocks he's spun re the phone and his conversations. No wonder you're beside yourself if that's the story he expects you to swallow. He does realise that he married a very intelligent, extremely savvy woman who's not about to believe that crock of shit, doesn't he? I mean - who the hell would go the trouble of setting up an entirely secret phone just to have totally innocuous conversations with an ex-work colleague several times a day? He doesn't have any bloody idea what he's doing to you when he's trying to convince you to buy that. I know that you want to believe it - you don't want your life turned upside down and if there's even a slim chance that you could be wrong, you're going to grab that straw with both hands and clutch onto it as if your life depended on it. You must be in turmoil emotionally. I don't know what to say 
I know it's so very hard but you must try to hold onto the truth as you know it - he lied and it's the lies that are fucking the relationship up, not the truth behind what happened. His explanation is very unlikely but it could be that the truth is not far behind. Unfortuantely, whilst he continues to lie, he's playing with your emotional state of mind in order to just brush this under the carpet and protect his own skin.
If it helps, I'm like a dog with the proverbial bone at the moment with my H - I won't let go of the fact that he lied for 6 months and tackle it every single time we talk. I just keep getting the same response "I didn't want to upset you". It's driving me crackers! Something I read on another MN thread really hit home today. It was on cwtchy's thread (although I realise the situation is very different from yours). It was posted by an MNer called Midwife99 I think. I'll try to find the quote and post it - basically she said that whilst our motivation to seek the truth may be borne out of frustration and a need to clear the air, the cheating party's perception is that we're clingy and we need them. It really struck me that my H's perception of me is as a paranoid wreck trying to cling onto him desperately. I mean, that couldn't be more wrong but really that is how he perceives me when I bang on and on about the same old crap every day because I want an answer that makes sense.
I'm going to stop wittering now because Christ Almighty I've gone on a bit! I'll see if I can search out that post and post it here for you - it might help you have a lightbulb moment like I did today! Hope you're having a nice 
- after this mammoth post, I think I'll join you....