I've got sciatica too (it was the reason I started SW a few weeks ago).
I get pain all down my left leg - awful nerve pain. It started last October, and my Physio thinks it's a combination of a big tum and too much walking jarring my spine.
It does get better! - I got referred to a physio by my GP and she's recommended that I build up my stomach muscles and do lots of cycling/swimming. 3 months on and it's far less painful and the pain is less frequent.
Good luck with yours - I would recommend pushing your doctor for physio as mine has been great and it was such a relief to find out exactly what exercise was still ok.
Breaststroke will make it worse because it puts your back into extension which will nip the nerve. Does lying on your front relieve the pain? Does it go when you lie down?
I was told that breaststroke is fine - but this may be due to the position of my sciatica (right at the bottom of my spine).
According to my physio no further damage will be done by non load bearing exercise (like swimming and cycling) - although it does increase the pain whilst I'm doing it, I think it's worth the pain! In fact she does a 'manipulation' on my which is similar to breaststroke (she moves my arm up and down whilst I lie on my stomach).
I find the pain worse when I don't exercise, as my hamstring starts to tighten and I move less freely.
I find sleep easiest by lying in a foetal position with a pillow between my knees, or lying on my back with a pillow under my knees. I guess it's all down to the individual injury though.
Good luck with your recovery!