Just to update on this, I ran mine down to the yellow light yesterday. A mixture of London traffic and fast motorway (slightly over 70) gave me 472 miles, and exactly £100.00 to fill it up at Sainbury's (had to squeeze the last £5.00 in). Got a long leisurely trip oop north next week - going to see if I can get 600 miles this time. I reckon it should be possible.
That's a '99 TD5 auto with egr blanked and standard tyre pressures, running on Sainsbury's doozil.

At £1.20 per litre i'll fill my '99 TD5's tank at £110 and get down to 640 miles before light comes on!