Reading your post. Please make sure you do have natural gas, just because you have piped gas doesnt make it natural gas. majority of the time if your on a holiday park it will still be LPG even if piped as it will come from LPG tanks sited somewhere on the park. This is exactly the same as bottled gas just bigger versions.
Check with the park first, but I`d put money on its LPG. Its a common mistake a lot of people make.
As for fitting a new fire, once you`ve checked that you have the correct gas supply you can then look for a replacement fire, Widney and Spinflo are you best options for caravans, stay away from flueless fires and I wouldnt use any other type personnally.
Hope this is of some help