They are as reliable as any other rumour source!
Some shit is reliable, some isnt!

To be honest, there would be no reason to make this up.
Simm is a good actor, and reasonably popular but he hardly has the star power to make people say "Holy shit, its John Simm!"
If they were to say it was gonna be someone like Jude Law or Ewan Magregor, then I might start to call it into question!

Snarf is shit. His death will be rejoiced!