A beautiful hike. In terms of the walk up, one of the most scenic I have done. It is a very wide river bed with huge trees towering over either side and because the river follows a straight course, more or less, you can always get views back and forward.
Views at the actual summit possibly a little anti-climactic due to localised smog and the surrounding topography being quite flat which means you are basically looking down at mostly agricultural land. On a clear day you would be able to see across the sea towards Phi Phi and although I was able to make out some islands, the smog definitely restricted visibility. The views were still very good and I would rate 7/10, I was just expecting an 8 or 9. I rented a motorbike from Krabi Town and there was often the smell of burning in the air whilst driving. You don't go to the actual summit of Khao Phanom Bencha as expected, it'd be a detour for no views. Overall I would rate the whole day as an 8.5/10 and the trail as 'hard'.
Hard - physically demanding, steep hillsides, difficult terrain throughout, for those who have experience in moderate trails already.
It turned out there is a preloaded track on backcountry navigator which I noticed the night before, as I was downloading offline maps for the area. I should have checked that in the planning stage. Without that track I'm not sure I would have made it to the top. Certainly it would have taken longer. You start from the dam as mentioned in previous posts (which is now repaired). The track tells you to follow the river course until the campsite. However the trail is not obvious. I first tried a trail to the right (when facing the mountain) near a broken portable toilet. Don't bother with that. There is a trail but it is never clear in the first place and eventually disappears without you having made much progress. The left side of the river (when facing the mountain) seems more likely for trails. On the way back down I did follow a trail on this side which brought me out quite far from the river, it might be worth taking that route if you want to be hasty. And on the way up I did join a trail on the left of the riverbed a couple of times. But I really think it is better just to walk straight up the river bed for the most part. There is a bit of hopping from rock to rock and you will get your feet wet but it's the most direct route, the most scenic, and it's not as slow going as you would expect - it's mostly walking up waterfall cascades where the rock is flat and not so steep. I think possibly when you get closer to the campsite there is no trail on the left of the river, at least I couldn't find one going up or down.
When I was doing this hike the water level did not seem that high, so the above might not be advisable after prolonged heavy rains (August/September?).
The backcountry navigator track only really became a necessity once I got to the camping area. Before then the track isn't clear if you should be on the left or right of the river or in the riverbed itself. But once at the campsite the river forks into two (this is the first fork in the river you will come across, after about 2 hours) and you would not know where to go without the track - you should take neither fork and instead enter the jungle in the middle; this is the first time you will be in forest (assuming you hiked up the riverbed from the start).
Even with the track I did not see the path because it was overgrown at the entrance. So I took the left fork knowing that later I would have to cut back to the right to try and find the path. However the track is there, you just need to take a few minutes to find it, and once on it it is only overgrown for a few metres. After, the track is easy to follow right to the summit. This is clearly a trail that is in current use and there must be fairly regular hikes to the top, judging by the condition. I cut back another 1km or so further on and that wasn't easy without a machete but I found the trail where the app said it was going to be (a relief as up to this point I had not actually been on a proper trail so was not sure if it was going to be easy going or even exist). Until you get to the ridgeline it is very steep without any respite.
I think this is a hike you can do without a guide, up and down in a single day. I started at 11am and was down by 6pm, and I wasted 30 minutes at the bottom looking for tracks and another 30 hacking through jungle just after the campsite. I walk faster than the average person and also jogged back down, but I think starting at 8am, most people could get up and down before 5pm. I wouldn't advise anybody to try this hike without a guide if you do not have good experience using GPS apps and trekking in Thailand/jungles, because it's a little too long and a little too physical. If you want a hike in Krabi you can do yourself which actually has better views than this one, try the Tubkaak Hang Nak Nature Trail.
If you want to test your hamstrings without spending a full day doing it, try the 1260 steps to the top of Wat Tam Suea. (That's Phanom Bencha on the horizon).
When you get close to the ridgeline you start to get viewpoints opening up and there is another campsite before the final push to the main viewpoint. You can see Phanom Bencha's actual highest point behind you. There is still running water here and for the whole hike you do not really need to carry a lot of water. One small bottle would be advisable only for the jungle part after the first campsite and for the very final stretch after the last campsite. You can fill it up each time you reach a stream.
When at the summit there is 1 sidetrail for 50 metres or so which should not be missed as that's an impressive viewpoint. But the main event is a bare outcrop of rock with sheer drops beneath you. You can see the dam below you where you started from and track your path up the river and you can see the fork in the river where you entered the jungle etc.
Here is my GPS file for the hike. Don't pay any attention to the distance (24km), my GPS kept jumping around. Overall distance for round trip is probably not over 13km. Start point is 150m elevation and summit is 1300m, so total elevation gain will be about 1.4km.
The aforementioned Bang San is indeed the start place for the hike and here is the name in Thai if anybody wants to try searching it, but you just have to head for the dam really, you can forget the village names.