I believe you have to give permission to each profile to view each VIsual Force Page. It sounds like there is a profile that you have not given permission to view that VF page. Are all the effected users using the same profile?

No, the affected users are on different profiles - bizarrely, those problem profiles can see the iframe if they use a different pc.

So it is only for specific users on a specific PC? Did you try a different browser. The only thing I can think of is that it is a caching issues. I would try to clear the cache on the browser on the problem computer.

Shot in the dark, but does your iframe actually have a semicolon in it after the src attribute? It shouldn’t — but this could of course just be a copy-paste error.

Also, I’d recommend relative URLs whenever possible to enable portability between Sandboxes / Production and to protect you against SFDC pod transitions, e.g. if you get switched from EU1 to EU6 or something like that, just make sure to prefix the VF Page name with the namespace it is included in, so your URL would be this instead:

/apex/crms14__rdAccount?&isdtp=vw&id={{{$Param.id}}}

Great thanks, changing the URL seemed to do the trick.