Stephen Mangan (Guy Secretan) actually did eat the "expensive" coffee (and the tea, and he doesn't recommend it!)
Some of the weird occurrences in the "theatre scenes" were based on actual events which took place during a real live operations which the actors observed before they began filming. For example someone's mobile phone went off, someone was practicing their skiing and "Stairway to Heaven" was playing on the stereo.
When Martin was on the computer looking up what his old schoolmates had achieved; Oliver Chris had a bad hangover and felt very sick. That was why there was a cup on the desk just in case he needed to throw up.
The door which Guy walks out of after Caroline has threatened him with a poo-tipped ski pole is a cupboard with the number 22 on it to look like the front door.
Oliver Chris and Katie Lyons would both go on to later to star on Bluestone 42