Now I'm pretty sure that tap-tempo delay devices weren't available in 1972, or even in 1985 (I have been listening quite a lot to a live Martyn album recorded in 1985). So how did he get music and effect so well matched?
It's down to learning to play in time - you figure out what the tempo is and set the delay time to a note value (16ths, eighths, etc) then play along in time to the delay. It's a skill you learn - pretty much like learning to play in time with a drummer. As with a drummer, you can also learn to push or pull the timing by playing slightly ahead or behind the beat.
I've worked with drummers that could play exact tempos without, but commonly pro drummers will use a click to keep the tempo - and once the tempo is set and delay times are set correctly, everything clicks into place ('scuse the pun).