Found it!!!!
It's called a Knightson Computer. Simply select the computer that matches the cruise TAS of the aircraft that you fly and it's very simple to use.
www.knightson.org.uk
MODS - I am in no way connected to this company other than having just purchased a computer for myself! I believe it to be a benefit to the GA community! (Luckily they live about 10 minutes down the road from me, so I could pick mine up!)
TBK