Generally, when you get on, there's unlikely to be a guard sitting in the compartment, so it's very unlikely to be a problem. I've not had an issue when we've used up more than the 2 official spaces before. I've also had my bike in the carriageway on many occasions, and not had any complaints from the guard.
Of course, from a personal point of view, 5 bikes already there does make it more difficult for me to get my bike on at Luton!! Just make sure you stack them neatly, in an alternate fashion inside, then there should be enough space. Don't forget the comments I've put back on page 1 re the platform length at Loughborough.
Anyway, as I've got a 1st Class ticket (seeing as they were the same price) plus a bike reservation already, hopefully this will give me some leverage when I get on!