How to phrase this?

[joeylittle]

The only reason we might be considered to be on periodic tenancy is because we have not returned the contracts, not because of anything the landlord has done.

I think @Sideways point was that it's the landlord's responsibility to get those contracts signed and returned. His obligation to come round and collect them, not your obligation to get them back to him. So, the landlord not having the contracts is a result of his negligence, not yours.

That's not just semantics - he's the one initiating the contract, that's what makes it his burden.

ETA: You only need to be concerned with your own ethics. And it wasn't your intention to screw landlord over by not returning the paperwork - you didn't have malicious intent, you just didn't get it done.

Honestly, I'd call it a lucky break and take it.