I'll post this as a guide to anyone disfortunate enough to have to go to this place on their own. Singkhon Immigration is the Prachuab equivalent of the "One Stop Shop" in Bangkok ... one very long stop ....
Singkhon is for foreigners that need visa extensions or renewals, you can't do in-outs here. Ususally after spending most of the morning with the sour faced officials in the Prachuab Labour Department who be-grudge signing the stack of papers that you or your company have nursed over the previous months just to get you the precious little blue book that permits you to stay here legally without resorting to selling gem stones over the internet, you'll need to go to Singkhon.
Directions first so you dont, like me, drive into Burma and have people bearing down on you with AK-47s, politely telling you to turn round in the only language they understand. Leave Prachuab south for around 6km until you see a Mitsubishi dealership on your right, take the next u-turn and double back. Then take the next proper road on the left, just after a temple called "Wat Nong Bua" . Follow this road and take the left turn when it forks, follow this for around 5km until you come to a T junction. Turn right here, past a temple on your left (pray you dont have to ask for directions at this stage as most people by now have never seen the elusive "farang") and Singkhon Immigration (the only English signpost for 20km) is on your right.
Park in the shade because nothing happens fast here. Eventually you may get the visa or extension you wanted and be able to navigate your merry way back to civilization ... or Pethckasem road, and head north for Hua Hin ... good luck and may the force be with you ...