Yes, had it this year. You are right - unbelievably painful. When it happened it was like my brain had maxed out in terms of the pain it could describe, but started to feel very sick and unable to move or even stand due to the pain.
I took painkillers to get to sleep, I woke up in the night in excrutiating pain and had to take more in the middle of the night. This is from someone who can tell you all the times in their life that they have taken painkillers (headache aged 19, fractured rib in late 20s, knocked off bike in my 30s).
There are some NHS exercised to do for Siatica. Do them daily, even if they hurt. My understanding is that most pain is your body telling you to stop. Sciatica is a trapped nerve, so the exercises aren't working the area of pain, but moving your back around to release the nerve pressure, so keep on going no matter how much it hurts.
Also get back on your bike. Any exercise that you do regularly and therefore have strong muscles and a stable posture for is good. You need to keep your back moving and keeping muscle tone where it is already good.
The problem is that there doesn't seem to be anything that you can stop doing to prevent it happening again. I aggravated it a second time by leaning and twisting to put a bike lock on my bike. The only lesson really is that if you have put your back under a bit of stress (lifting, riding up steep hills), be very, very careful for the next few days and don't do any back twists for any reason at all.