Thanks everyone - I'm going to skip going to Division 1 in that case and just get a reentry permit at BKK on my way out. Ideally a multiple one so I don't have to go through it all several times. The main thing I wanted to know is that it's possible to get a single reentry permit again so worst case scenario if they don't offer a multiple one I will just get a single each time.

If I have a 3 month category B work visa issued on June 1st 2017 and then enter on say July 25th for the first time, how long validity will I get on my entry stamp? Is it until the visa expires (so September 1st approx) or 90 days from entry (so October 25 approx)?