Tenants arrested for growing cannabis - what should I do?

General Discussion

My tenants have been arrested for growing cannabis. Small scale as far as I know. They did't tap the electrics and had a few plants.
I've only been land lording for a few months. What should I do?
Should I evict them?
They are 4 months into a 12 month AST contract. I have had them sign a section 21b form the day after their tenancy started.
My inclination is to evict them and I feel want them out immediately.
Do I need to give them a months notice?
Any advice anyone?

Thanks,
Colin in Leeds