Update -
Job done - 3/4 of a mile of double banked former railway line gassed.
After contacting the guy who ran my course several years ago he confirmed my suspicions that no mask was 'needed' as well as a bit of an over the phone refresher course - to highlight the main and important factors while using phostoxin.
Dry, overcast weather with a light breeze made for decent conditions and we worked the bank so we were always upwind of the gas.
A system in place whereby I applicated the poison and a second man backfilled holes with sand after placing 2 sheets of newspaper over the hole itself.
Everything went well - used nearly 300 tablets and not so much as a headache for either of us.
Deffinately common sense needs to take presidence and its easy to see how some people may come a cropper to the stuff.
The one strange thing was this - I had a very very acute nose for the stuff, it releases a powerful smell if you get in the way of the fumes and therefore a slight whiff and I could move out of the way, take a breath and move back when the breeze had cleared the air. Very easy, safe and simple - never felt funny, nausea or headache at all.
But
The guy I was working with couldnt smell it he didnt have a cold or anything but seemed to lack the power to identify the smell. Therefore if I had not of repeatedly move him backward into clean air he would of just stood digging in toxic air, completely unaware?
Maybe some people can smell it better than others, I could personally smell it very easily and this allowed us to work safely and breath very little, if not none of it in. Im sure my apprentice would of being going home rather ill if he had been on his own.
So IMO be careful and if I was doing it full time I would certainly invest in some technical breathing apparatus.