I have extensive experience with TDI fuel systems, especially A2 TDI's and can offer you the benefit of some of my experience and knowledge.
IMHE a blocked fuel filter will not stop a TDI from starting. The first symptoms of a blocked fuel filter will be hesitation at high fuel demand ie accelerating up a hill. If the car didn't exhibit these symptoms when it last ran then changing the fuel filter probably won't fix it. That's not to say that a new fuel filter might not help or is a waste of time. It's possible that a blocked fuel filter has caused excessive negative in the fuel lines such that air has entered the fuel system and reached the injectors. If this has occurred the car will not start even after the new fuel filter until the air has been bled from the injectors. I once had a TDI fuel filter so blocked that the car was virtually undriveable....but it still started.
It is also possible that air has reached your injectors for other reasons that have been touched on in this thread. These might include running out of fuel due to faulty tank sensor and parked on hill; rodents eating fuel lines; decay of fuel lines due to abrasion; natural hardening of fuel lines due to diesel at connectors; leaking tandem pump.
I have experience all of the above reasons (some multiple times) for a non starter.
It's important to realise (so I'll say it again) that one air is in the injectors it will be almost impossible to start (even after the cause is fixed) without sucking the air from the injectors through the fuel return pipe. You are more likely to flatten your battery before it starts just by cranking. The TDI fuel system self bleeds very well when it is running but one air is in the injectors and it is stopped it does not self bleed.
The process I would follow (some of which you have already done but understand I am also writing this for future readers) would be:
1. Check all fuel lines in the engine bay for signs of damage.
2. Pay close attention to where rubber fuel lines connect to metal. Try twisting the rubber on the metal spigot without undoing the clip. If it moves then the rubber has gone hard and that is probably where air is getting in. Tightening the connection will probably not help, new, supple fuel pipe is required.
3. Run your fingers underneath the tandem pump where it connects to the engine block. Any sign of diesel here is evidence you need a new tandem pump gasket. A leak here will stop it starting.
4. Check fuel pickup in tank is not plugged.
5. Replace Fuel filter. Though this is not the likely root cause, a blocked filter will exaggerate the leak points above.
6. Scan for possible electrical causes.
7. Bleed fuel system as follows: Disconnect fuel return pipe in engine bay. The easiest place to do this is on the left hand side (when facing engine) connection. Take the rubber pipe off the metal pipe going across the front. The return is the lower metal pipe. Connect a vacuum device to the metal pipe and apply suction. I use something like this:
1 x Brake Bleeder. 2 in 1 brake bleeder and vacuum pump tester tool kit. 1 x Reservoir Jar. Accurate vacuum gauge: 0 - 30inHg ( 76mmHg ). 2 x Plastic Cover.
www.ebay.co.uk
Once vacuum applied get an assistant to crank the starter for 10 seconds at a time while constantly applying more vacuum. If you have used a clear pipe from the kit above, you will see air in the fuel. On second or third attempt it should splutter into life. Make sure the assistant switches it off before the canister is full of diesel.
8. Once it starts, run at tickover for a few minutes for the remaining air to clear.
Don't assume that an absence of external diesel means there isn't an air leak in. The fuel system is mostly under negative pressure so air in doesn't necessarily equate to diesel out.
If you find a possible source of air in the fuel system then eurika! fix it, start as above, problem solved. If you don't find any possible sources of air in the fuel system then starting as above will still likely work but you will not have resolved the issue. It's possible that the new filter will create less negative pressure and so the problem may appear fixed for a while but will eventually return.
If trying to start as above fails then air in the fuel system might NOT be the issue and you might have to consider other possible causes like relays or engine timing (has the cambelt been changed recently). However in 95% of non starting cases I have experienced, following the above will get it sorted.
Hope this help you or anyone else who has a non starting TDI.
don't know whether to wait for a deproman drain screw or have a go anyway as i have nothing to lose. Need a lot of molegrips and a coffee break...why is the fuel filter where it is ????? 
.. have looked all around the tandem pump also ... Hopefully in the right place and cannot see any significant oil, diesel leaks ...
, looks simple to do apart from the drain screw issue and also keeping the lines airtight.
Have to order it - ECP, GSF earliest Monday delivery
