I solved my old oven cleaning problem (and believe me, mine was bad as I can no longer get down to floor level to clean it) when I was leafing through one of those coupon business saver newsletters.  There it was in black and white - someone local who cleaned ovens and the coupon was for a 50% discount.

I can't remember what it cost, but really does it matter?  I phoned the number and the guy came out about a week later.  He had his work cut out but he did a great job with a small steam cleaner, basic eco friendly cleaning materials, elbow grease...and no harsh chemicals.

After that I have put one of those silicon sheets down under the element to keep the base clean.  When the oven needs a good clean again I will be phoning the guy to do it.