I have a dilemma where I need a GPS to assist when I am off road halfway on the side of a hill in the middle of no where MTB also need to plot record heat rate cadence etc, I already own and in fact selling a Satmap because I am thinking that Garmin 800 is the way forward to do all this. Am I correct or should I keep Satmap & get a smaller Garmin I would rather it be all under one roof also struggle with the Satmaps website downloads/uploads etc.

I would have thought Garmin 800/810 would be your best bet

Or something like the Etrex 30, Dakota 20 or Oregon 450. They will do maps / navigation, and plot your heart rate / cadence (if you buy the sensors).
If you want to use OS maps, I think having a bigger screen is good, eg the Oregon.

Everything you could possibly ever want to know

http://www.dcrainmaker.com/product-reviews/garmin

Thanks guys I will look into the reviews