Internal server 500 and this is the only text I could get back from PHP running on IIS.
It was being caused by a disabled SOAP php extension and I (eventually) tracked it down firstly by ensuring the error_log was specified in PHP.ini and pointed to a directory that was accessible by PHP. I then ran the troublesome piece of code and got "SoapClient" is an unknown type in this error log.
I don't know why this wasn't picked up at application level and sent back to the client in the form of a useful error. My try/catch blocks and everything were bypassed, I guess because this was right down in PHP.
I added in the extension, re-started php-cgi.exe to reload the PHP.ini and it was all well again...
(This all took me 2 hours but should have taken me 10 minutes!)