for anyone who is interested, i finally got zac to have a sleep in his porta cot
and the way i did it was to actually get in there with him and show him it was ok to be in there. We read books, sang songs and drank some milk in there and then each day after that he had a play in there and then finally about a week later he has had 2 sleeps in there. I'm so proud of my little man and now i'm really looking forward to our holiday knowing he will be relaxed too when we all go to bed.
thanks everyone for your advice.