Mizter T wrote:

Though with National Express a Cambridge - Heathrow return costs £33,
whilst an open return costs £35 (you need to ring them beforehand to
confirm your return journey so they allocate a seat for you).

I don't think you have to allocate a seat. I have never done this in
many Cambridge-Stansted coach trips. If the coach were full, though, I'd
be in trouble. Has never happened.
--
Michael Hoffman