I've been meaning to post this question for a while and kept forgetting. I was due to fly back to the UK from Munich on the second of two LH flights that day which I discovered was cancelled when I arrived at the airport. I was offered a hotel overnight and a seat on the next morning's flight. I did briefly try and convince the agent to book me onto the KL routing via AMS instead but was repeatedly told "We are unable to book you on that route" even though I could see there was space available.

So how should one approach this? Is there a piece of legislation I should quote? And was I within my rights to expect the KL routing?