Thanks again, Calvin! You were absolutely right - your advice was spot-on. My Zurich-based solicitor has confirmed this clause is in breach with Swiss federal law and therefore null and void and not enforceable. He also confirmed that any Swiss canton cannot deviate from this legislation.

Do you know what the most shocking aspect is? At the time of signing the rental contract, the landlord said to us. "Trust me, this is a standard contract. I should know as I am a solicitor." Yes, I am not kidding you - the landlord who included this clause in the rental contract is a Swiss solicitor. Although business has been week recently, by own admission. I think I now know why that may be the case. My suggestion would be: back to law school...