Only just started happening, if I'm for example approaching a roundabout in 5th and knock it into neutral the engine cuts out. Other times if I'm going from 5th to 3rd or 4th the revs drop and it almost stalls but when my foot comes back up off the clutch it's okay.

It's very random and sometimes it's fine. I think the times it does it I find starting taking an extra crank or two.

I've searched around and it seems it could be air getting into the fuel line - is there a way of tracing the location? Any other help would be much appreciated

PS - I should say it's a 2.5TD.

Air in the fuel lines can be seen by the precence of white smoke from the exhaust..
The fuel lines from the tank can be easily seen under the removable cover under the drivers seat.
On a 300TDi they pass under the drivers footwell by tthe gearbox tunnel and up to the pump (beneath the air filter) and up to the fuel filter before going to the FiP.
The return (that joins/loops across the injectors) goes back to the tank alongside the feed.

Look at all the pipes, wipe them with your hand, if there is a leak you will feel/smell it.
Check the lifter pump (activate it by hand) to see if Diesel is being pumped into the filter.
The lifter has a wee filter in the top, see if that is clear too.

Also check the little fuel pipes that join the injectors, these have a tendancy to go brittle and split

Thanks both, I got my mum to look and when I started up there was white smoke. Then when I reved it up a cloud of white came out. Checked all the pipes and no sign of fuel on them. Also the injector joins look good. Tomorrow I will check the pump. I'm not sure but I think I can hear a slight airy ticking sound from the fiel pump area bit it's hard to pinpoint.