I'm sure that I have already given you these details in a previous post, but Pattaya (U Tapao) to Phuket is only flown by Bangkok Air
Daily via Koh Samui
UTP USM 1210 1310 PG 253 Y4 M4 K4 V4 B4 AT7
USM HKT 1400 1450 PG 253 Y4 M4 K4 V4 B4 AT7
And direct on Wed, Fri, Sun
UTP HKT 1720 1850 PG 272 Y4 M4 K4 V4 B4 AT7
I consider the question should I book now or later, a rather silly question. If you know when your going to travel then book it. If you don't then you can only blame yourself when the flight is full.
See
http://www.bangkokair.com