I had a look on Ebay and found a double action gravity feed airbrush for £8.95!
Just used it for the first time and it works perfectly. Build quality is not bad at all but at that price even if it fails they are cheaper than a replacement nozzle for the Aztek.
Only issue was the connection as the adaptor just had a push-fit plug designed for a large-ish hose. I had an old Badger plastic hose so I soldered the adaptor into one of the ends and it works perfectly.
While I was at it I thought I'd buy some modern varnish so also ordered some Mig matt. I can't believe how much easier working with this is - no thinning, no decanting, wondering whether it will clog, just stick it in and spray (I used to use Humbrol gloss and matt cote).
I'm going to start a collection of colours next and maybe switch from brush painting completely - seems so easy compared to trying to spray thinned down enamels (which is what put me off airbrushes in the past and why I limited its use to just varnishing).