Playing in sessions is really good practice for playing in time. Sit on the edge of the session and play quietly (not always easy with a concertina). Don't worry about playing all the notes, but concentrate on playing the "strong" notes in the tune and keeping up with the others. As you get more confident you'll be able to fill in more notes until you find you're playing the whole tune.
A common problem with novices is that they struggle to fit all the notes in, and this holds up their playing. I actually think its more important to keep the pace and rhythm going than trying to play all the notes. At a more advanced level, you find players struggling to add ornamentation which holds up the tune. Concentrate on getting the basics right, and you can add in more as your level of ability increases.
This is great advice. It'll help you to start keeping in time, and is a great way to start to pick up tunes by ear, too! The most difficult thing about learning to play an instrument is stepping into that group thing, where everyone is going to carry on whatever you've just done. Learning how to keep up (this includes dropping out til you recognise where you are, or playing only the occasional note or short phrase each time you hear it) is so hard! At home the temptation is always to stop and put your mistake right, or go back to the beginning, but some practice sessions at home should include "carrying on with the rhythm regardless of what notes you're playing", maybe the metronome or playing along with the CD can help here (and I should take my own advice! ).
All the best