OneChordTrick wrote: "I've focussed on melody plying but hoping to pluck up the courage to join a local pub session. ..."
"... I've got the music for various tunes which generally list the chords above the notation but how should I play them?"
You said "pub session" so I'm assuming it's Irish traditional music? If so, that changes (and complicates) things considerably, compared to say oldtime jams or something.
For starters, chordal accompaniment in ITM is a controversial topic. Here's an older MandolinCafe thread that goes on for 3 pages about whether or not chordal backup of any sort belongs in ITM 
(it's about guitars but the concept extends to any instrument that's playing chordal backup in ITM).
Some people, including myself up until recent years, 
believed that ITM didn't benefit from *any* type of chordal accompaniment and that such accompaniment was unnecessary and disruptive.
Obviously, times have changed, there are now guitars and bouzoukis in sessions, sometimes playing nicely... but sometimes just crudely banging out chords, 
I will never get used to that (in fact that's the reason I quit playing at Irish sessions in the 1970s, I'd had enough of the new crop of crude heavy-handed chord-thrashers who IMO 'ruined' every tune they got their hands on, but I digress).
However, accompaniment *can* be very lovely 
such as (IMO) some of the wonderful music by Martin Hayes and Dennis Cahill for example. Of course the typical pub session isn't going to be that graceful, but (IMO) it's still something to aspire to.
"... Obviously the simple answer is "follow the others" and it depends on the tune but are there any guidelines?"
#1 guideline: Absolutely *no* bluegrass-style chop chords. Chop chords just sound weird and they don't help the music at all. Playing bluegrass mandolin chords is not a good way to make friends at an Irish session. 
I'd say, if in doubt, just play the melody. That's likely what the majority of other players there will be doing, and that's fine, it's expected and normal.
If they're playing too fast for you to keep up with (when playing melody), then play about *half* of the melody notes (seriously!), 
just hit the important melody notes and leave out the others. Chances are it will still sound ok, and probably no one will notice (or care) about the 'missing' notes, as long as your timing is good.
If the session happens to have a talented guitar or bouzouki player who plays with finesse rather than thrashing everything into oblivion, then study them, listen and watch them, see and hear what they're doing, learn their style by just kind of letting it sink in by osmosis over many months if possible. Not sure how much of that would transfer over to mandolin though, given the mandolin's higher pitch range, but it might be a starting point for ideas I suppose (arpeggios and whatnot).
Hopefully other people here will have some other (probably more useful) ideas as well. 
