Thankyou so much for your reply. I talk all the time with my husband, we've got so good at talking over jealousy issues (usually mine admittedly) over the years. I've been working on myself an awful lot over the last couple of years and my jealousy has become much less, and neither of us ever display unpleasant jealous behaviour, we just talk it over and ask questions and try to do whatever it takes to feel secure again. Usually for me thats just being allowed to ask as many questions as I need to and have them answered freely.
I have been telling my husband about every feeling I have that pops up and whatever goes through my head, and we're both feeling really happy with each other but we both have feelings of "Are we completely crazy? What on earth are we doing?" sometimes I think.
I have spoken to the other guy about my feelings for him, I was worried about potentially hurting him if my husband and I decided at some point that this wasn't actually a good idea. Thankfully he replied that he was flattered, was having fun with me and enjoying talking to me regularly and is attracted to me and would love to play with me but doesn't think he has any deeper feelings. This is a guy who's heart I broke once so this is a huge relief to me that I'm not being totally selfish by getting involved with him again. I said before that he is already involved in a relationship with the wife of another friend for the last 5 years or so. Its not like either of us actually has time or opportunity to explore much more than a friendly flirtation but I guess I still have this nagging feeling that what I'm doing is wrong, particularly because he is someone I have always cared so much for.