I remembered reading a sign on the cove of Arriva's Sapphire buses that they have a no-quibble money back guarantee, has anyone actually taken advantage of this?

According to their website 

Our Sapphire customer charter sets out our commitments on things like reliability, punctuality, frequency and cleanliness and guarantees you your money back if you’re not satisfied

Clearly from other threads, and my past experience, they don't live up to this, so I'd be interested in seeing what would happen if one tried to claim a refund.

They still do it, but it was regularly abused over trivial things like drivers not wearing a tie etc, so understandably it got swept under the carpet.