Not an expert, but two things come to mind.....

1. Bike fit - saddle height, setback, stem length; to upright leads to extra seat pressure, need to balance weight between seat, pedals and bars.

2. Change saddles - Brooks aren't magical and may not work for you. Search the net and the forums, lots of information concerning Brooks saddles.

Also, sciatic nerve problems can be very painful and would severely impact any ride. You could try and sort this out yourself through trial and error, or, seek professional help through a qualified bike fitter. I believe this is standard advice, but it could really help you.