Thanks for the replies guys, here's a link to the circuit i'm using
http://solorb.com/elect/pwm/pwm1/
What i wanted to do is replace the VR1 pot that controls the speed with the output from one of the channels of my receiver. I realise that there will need to be another circuit to convert the receiver output to a suitable format, just don't have a clue what it should be. I know it'd be easier just to use an ESC but I'm interested to see if there's a way of doing it myself. feel free to say if i'm asking the impossible!!!