Just an update to wrap this thread up.
1. The air valve was on too tight for even the correct size Allen key, so ended up loosening it with a small wrench.
2. Although the O ring intially swelled up, blocking the airway it was actually fine after a couple of days and not the cause of the pulsation, which only occured when another medium (paint, thinners, water etc) was passed through.
3. It turns out that I had not thinned the paint enough and it had build up, restricting the passages. I resolved this by sunging the head assembly and front of the main airbrush body (air valve cautiously removed) I decanter badger airbrush cleaner for a for hours and removing and remaining loosened paint with some pipe cleaners.
4. Airbrush works fine now without any pulsation (tested with water only). I am very relieved and thankful, I was prepared for a long and expensive solution!