Thanks for the reply,
I just recieved some advice from an alternative doctor, Dr. Jim Occhiogrosso, who stated:
"If your entire butt area was numb it would indicate that you are either impinging on a nerve or preventing blood flow to the area. Either is not good. Suggest you put a time limit on sitting and get up, walk around and stretch every 15-20 minutes for a minute or two. That should help. Also you may be able to find a chair sized donut shaped blow-up pad that you can sit on, similar to those used by women after birth, or for rectal surgery to take the pressure off you groin area. You might find that at a medical supply store."
His website is http://www.prostatehealthnaturally.com
for anyone who's interested. I don't know much about him other than having read 2 or 3 articles he's written for my favorite health newspaper.