Thanks for clarifying the exact situation.
It is strange, because of the inconsistency.
It may be something to do with this.just another point the form is located in a sub domain https://secure.westwalesrallyspares.co.uk if that makes adifference
I suspect it is a security issue of some sort on the server.
But, there's no obvious difference between the testalert and your form submission that would indicate the cause of the problem.
I found a PHP/PEAR bug report here: http://pear.php.net/bugs/bug.php?id=4241
It seems to indicate that the bug is that the exact status (cause of the error) isn't shown. A bug in reporting not a bug in execution.
In other words, if you had the updated PHP/PEAR you'd be able to see the reason the server is reporting this error.
Since this bug report is Aug 2005, it's possible that you can update to a later version of PEAR and be able to see the exact message.
To upgrade PEAR: http://pear.php.net/group/docs/20051104-sa.php
It looks like you need the MAIL package version 1.1.7 or later: http://pear.php.net/package/Mail/download/1.1.7
Once you know the underlying error message, you'll be able to figure out what the server is complaining about and take steps to handle it.
As I said above, I suspect it's a security issue.