'99 Mk4 Golf TDI starting issues

[Big G]

I have a 110bhp TDI (AHF) and it's very reluctant to start lately. It is serviced every time it tells me it needs one and has so far covered 260k Latest service was within the last 500 miles using all VAG parts.

I last used the car on Monday night and this morning I goto start it and it acts like a flat battery. I recently took the car to kwik fit to take advantage of their free battery check which resulted in a good battery and good charging voltage hopefully ruling out my battery and alternator. The battery was new probably 18months ago.

The car doesn't smoke when driven even when driven hard and doesn't drink any water or oil. The car also has good performance hopefully ruling out any compression loss, although I will get a CR check just to confirm that.

Even when the car is hot it won't start straight away like you get out to goto the shops and get back in if will turn over several times before eventually firing up.

A friend mentioned a 1 way valve in the fuel line would stop any fuel returning to the tank under the theory: it has heat. it has compression (not confirmed). therefore it's missing fuel. I would of thought if you've left it off for all of 2 minutes it can't of lost all of it's fuel in the lines.

Any ideas or further info needed?