Thanks for the reply Perk. I read that link earlier, but it was a bit like martian to me! Made limited sense in theory but didnt really help fix the problem, particularly as the content was ok for the browser under one environment, but not under another.
Just discovered what the problem was, on the plus side it wasnt my noob php code - yay(!) - but it was my noob shared hosting lol. The paramater I was passing to the script was a URL, the server has some form of 'hack' protection filter that breaks URLs containing other URLs - undocumented ofcourse
Quick .htaccess line fixed it, but it was pure fluke that I happened to read about a similar problem in an ancient wordpress forum post.
Starting to think that despite being new to all this VPS may be the way forward, fixing all the little nuances is a pain in the ass.