Despite the absence of any reply (not sure if this question was in the wrong place or just didn't interest anybody!) i have found an answer which i will post as I know from past experience that nothing is more frustrating than trawling the Net for something find it and then found its not been followed up.

I managed to achieve my goal by having the external shell script write out the value to a file in /tmp, the macro then read in the file to the current file , deleted the line and assigned it to buffer 'a' and then using the @ notation to pass to the : (go to line) command.

map ^[[O :!goto_func %^M :r /tmp/goto.mac^M "add:@a^M : !rm -f /tmp/goto.mac^M

still a bit cludgy as edits the existing file, (im thinking is there a way of reading directly into the buffer?) if anyone has a more elegant solution then please post it here.