So after a bit of investigation today... I reckon that if you don’t want your trailer to go off the slipway, you need about 5m of water.
If you don’t mind your trailer going off the slipway, but want your car to stay on, you need about 4.5m.
If you don’t mind heading off the slipway and around the bend, I think you could get down to circa 3.5m quite easily, beyond that you’re going for quite a drive.
These photos were taken at about 3.6m.
The bottom of the slipway is very gravelly / stony, any half decent 4wd should be able to find grip. It looks a lot rougher than it actually is.
Quite why they didn’t extend the slipway another couple of metres I guess we will never know.
For anyone who has come in at lower tide, how hard is it getting to the slipway? Planning on some careful experimentation on a rising tide...