TOPIC:

I've had a look, and I've absolutely no idea what's going on here. However, after you receive the error (so start the setup, and when you encounter the error) you can continue the install at

php_modules_test.php, i.e. www.s-c-s-ltd.co.uk/xerte/setup/php_modules_test.php

I am afraid the errors continue:

so obviously something about the set up not working

Forbidden

You don't have permission to access /xerte/setup/page1.php on this server.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

Ok, there has to be some sort of php error.
Can you plase change line 34 if config.php into

and try again please? (Just from the beginning)

Please make a copy of config.php first, so that you can revert the changes.

OK all done using the standard setup until I get same error:

Forbidden

You don't have permission to access /xerte/setup/file_system_test.php on this server.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

Is $development (in config.php) still set to 'true'?

Did a file 'debug.txt' get created in the same folder as config.php?

If $development is true, and no file has been created, can you please make sure that the install folder is writable? (You need that anyway dureing setup to generate some config files).

If there is a file debug.txt, what is in there?

Also, can you check what's in /public_html/error_log?