Who should you contact to become a part of a film crew?

Currently, there is a movie called Filth in pre-production and I would
love to have the opportunity to work on set, maybe as a runner. I was
wondering who i should get into contact with to make this possible.
The Production Company(Steel Mill Pictures), the Director/Producer(
Jon S. Baird) or someone else? There doesn't seem to be anyone else
that I can really find information to contact.