I worked on the Isle of Grain , and have also worked with Costains amongst other big construction companies.
I think the word / abbreviation "Sub" is related to "subsidise me" i.e. until next pay day.
To my knowledge guys would line up at the paymaster's window on site and sign for a sub (say £ xx) which would be deducted imediately from the man's pay packet on the following Thursday or Friday.
Unfortunately in many cases it wasn't usually as "acegardner" stated on a Wednesday - the norm being a Monday morning after blowing all his/their money on booze,the horses and dogs and the ladies and maybe, just maybe, a few quid sent back to Ireland.
It was always reconciled by " I Work hard I play hard ", or as another guy said to me " I always get it the wrong way round , why do I always pick slow horses and fast women instead of the other way round ".
Sub also called a "rub a-dub-dub" which I suppose was to make the situtaion sound more humourous / less embarrassing.
The Sub also provided a legit purpose in that a new starter could borrow the money up front to buy work boots,pay an advance on his accommodation (digs) or e.g. if he had been out of work for a while , but that type of sub ( like all the others ) would only be given if the man had already worked enough time, equivalent to the amount he was going to borrow.
On the subject of the Pubs - the Gangers generally dictated where you would do your boozing - the Gangers usually having a "deal" with the pub Landlords.