OK
Thanks for both of our comments.
cattledog you have solved one of the problems! the failsafe now resets when a pulse is received.
And When i can get my head around what Delta_G has said then i believe i will have what i need?
Currently i measure the pulse correctly, But when i turn of the transmitter the interrupt is called but nothing happens apart from a flashing LED. However when i turn the transmitter on again the interrupt is called and the serial displays the pulse again.
Should i have used something other than a while? i didn't want the loop to do anything other than wait for a pulse on RxThro OR a interrupt.