I am glad to say that I finally got a prototype up and running, so a big thanks for all the help and advice.
I made a major boo boo with my original design, as I didn't check the specs of the LM567, which is supposed to work at a minimum of 4.7v. 
Although it worked at 4.5v for me, a slight drop in the batteries would have rendered the device useless. Fortunately they do a low voltage version that works from 2-9v and I just had to change a few component vales, as the oscillator has to run at twice the frequency you wish to decode.
Battery life should be good as it's using about 1.5ma in standby mode, according to an online calculator this would give over 450 hours.
Regards,
Les
