Our eldest daughter always calls us Mum & Dad, but our youngest daughter calls us Ma 'n' Pa, more often then not.
My siblings & I usually called our parents Mum & Dad - but when chatting or writing to each other, always referred to them as 'Mummy & Daddy' - we still do, even tho' sadly we're orphans now.
My Mum always used to say "ask Daddy" & Dad used to say "ask Mummy" - so it always stuck.
Just taken a look on the other thread Quizzy & posted this link on there too. Wasn't sure if overdale was looking for formal or informal replies, but your answer was brilliant as always.