It's a difficult one really because if you're in a session then you ought to be inclusive because that's what sessions are, BUT it's bloody difficult when someone is playing out of tune/rhythm or in a totally different key. Saying **** *** is probably not a good idea, especially if it's the landlord!! I'd probably stop & say " excuse me, but I'm a complete beginner at this & I'm not familiar with playing with asynchronous rhythms, would you mind if I did this one on my own & maybe we can find something you can play along with later". Having listened to a fair bit of your stuff Raggy, I'm suprised anyone would think of joining in with a drum as you tend to do nice, quiet thoughful songs!
As a singer I get a bit bored in music sessions so I've got a drum, but I play it very very very quietly and only with the really fast, roistering tunes that everyone joins in with, it gives my hands something to do & stops me smoking so much!!!
We haven't seen you for ages, hope that's remedied soon.