Thanks so much for the advice. Bobsnobs, just read through your original thread so you know exactly where I am coming from! It's awful isn't it.
I think from the noise the flies made when we sprayed Raid fly killer directly into the vents there are an awful lot of them breeding in the walls, so I have decided I would rather pay and get a professional in to get it sorted. It will just take the edge off buying the van for me completely if I have to keep clearing them up for the next few months while we try and sort it ourselves. I think they must have had a problem at the dealers we bought the van from as there were some flies in it when we bought it but they were all dead and I never thought any more about it. I did see them on the window sills in a number of vans they had for sale, so I am hoping if we get rid of them that could be the end of it as I think the source was where we bought it from, not the site we have moved it to.
I'm determined not to let it ruin having the van for me though as aside from that I am thrilled to bits with it and the kids love it too.
Thanks again everyone - it's such a great site this as someone is always able to help and advise