I think 50mpg or more from the 1.9TD, whether 306 or 405, is pretty good. I got 700 miles out of 63 litres in my 405 from the last tank fill, mainly due to a 450-mile round trip to Bradford on motorways, going no faster than 70. The rest was all A/B-road work.
My current work route is taking me to Trowbridge in Wiltshire (70 miles each way daily), which encompasses a real mix of roads from 10 miles of single track up to the A303/A34 dual carriageways. Like Plumb Bob, that's at the legal limit on all the roads (apart from the single tracks, where it's a maximum of 45 and more like 20 round the blind corners!).
The last partial fillup for this route I did got me 410 miles from 38 litres, which I make 49mpg. I'd probably get more if I took the roof bars off! Car is serviced every 5k (last done 2k ago) with new air, oil and fuel filters. Check the tyre pressures on a weekly basis too.
I had a 2007 Saab 9-3 1.9TiD (120bhp 8-valve model) as a company car in my last job, and driving the same way I was lucky to break 50mpg for a tank (according to the trip computer). The best I ever achieved was a run up to Dumbarton in Scotland, with a stop at the Lake District - the computer reckoned 56mpg for that one. A 2007 9-3 weighs a lot more than a 1994 405, which rather negates the benefit of direct injection! I'd also say the refinement of the engine when on the move is better with the XUD
If (rather than when!) my current 405s go to the Peugeot dealer in the sky, I'd probably consider a Mk1 Octavia TDI estate, if there were no suitable replacement 405s available...