The other morning my 58 plate 1.6 tdci cut out shortly after starting. AA came out and diagnosed air in the fuel lines. After bleeding the system the car started on the button and ran with no issues on my 40 mile commute. When I returned to the car 8 hours later the same thing happened. Fortunately I was able to get hold of a priming bulb, bled the air out (only took 4 or 5 squeezes of the bulb) and returned home. This now happens every time the car is left for more than an hour. Clearly air is getting into the system from somewhere but what is the best way to diagnose the cause? 

It's a pain to diagnose as you're unlikely to see a damp patch from a leak.  The air gets sucked in and the fuel just runs back to the tank instead of out the hole.

The best method is a very close visual inspection for cracks and holes.  If the fuel filter hasn't been changed recently, I'd be checking the connectors carefully.  Wouldn't change it just for the sake of it as they're so expensive on these.

You could also place your priming bulb as far back as possible on the intake line (probably near the cambelt cover from memory) and then pressurise the system ahead to see if that forces the fuel out anywhere.

If you do narrow it down to a connector that you suspect is the problem then the old school way of checking is to put plenty of Vaseline around it and if the problem goes away then you have found the cause.

3 hours ago, DanUK said:

Clearly air is getting into the system from somewhere but what is the best way to diagnose the cause? 

These cars, like most Ford diesels, have no tank pump, and no valves to stop fuel draining back to the tank. Since the tank is lower than most of the engine bay pipework, this is all under some suction all of the time, just a bit more so when the engine is running and sucking in fuel. So any tiny crack in a pipe or leaky joint will allow air in.

Like Tom says, trying to pressurise as much of the system as possible with the bulb might help. Otherwise it is mostly physical inspection, feeling metal pipes for corrosion or damage, a possibly removing flexible ones to inspect them properly, bending them to try to open up any tiny cracks, and looking for wear or damage.