The Punch and Judy is very obvious as it has a large balcony, but it also has a basement and a another downstairs bar so would be a good idea to arrange where you are meeting.
To get to the market take the tube to covent garden, exit the station to the right with the masses, you will see a flower stall and the market, turn right and walk around the market, then look up. You will see the balcony there are normally people juggling, etc. in the foreground. Enter just round the corner.
Hope you all have a great night, it will be busy though so arrange somewhere to meet (the balcony is always good)