I've got a work around for now which works for one of my clients - they have X number of tables for up to 4 people and Y number of tables for up to 6 people... So we've set the calendar up for 2hr slots with X number of tables available per slot for groups of up to 4, and Y number of tables available per slot for groups of up to 6 - for sports screenings we amend the slots to 4hrs. This could work for tables of 2 as well, but as its a pub they get more groups in so 4 and 6 works for them.