Is this statement that the server app pool crashes coming from them, or based on something you've seen?

Also, I'd be worried that PHP is becoming unavailable. PHP may throw a fatal error in response to a script becoming unavailable, but that should only terminate that script's execution. PHP itself becoming unavailable due to something done in an application really shouldn't happen, especially on a shared server.

When I last looked at Fasthosts the only way to achieve SSL on their shared servers was via their secured vault facility. This required having a second copy of the site in a limited space. So limited indeed, that we concluded that even ignoring the hassle of keeping two versions of the code in sync, it could only support small sites before the available secure space was exceeded.

To cut a long story short, at Kuroi there are hosts upon whose shared servers we refuse to work, because we know that we're risking a whole heap of problems unrelated to actually developing sites. Fasthosts is on that list.