I seem to have got to the bottom of this, so if it helps anyone else I'll explain.
Start up MAMP. In 'preferences' go to 'ports' tab and click Reset MAMP ports. Apache Port is now 8888 and MySQL Port is 8889 on my machine. Now I can view my
PHP files again, yipee!!
Let's hope it stays that way! and Lets hope this doesn't cause me more problems along the line, who knows?