low to high on the dipstick is 1.5 litres on a DE so it looks like you're using 1 litre per 1000 miles ... mine started using oil in a similar fashion at about 70,000 miles ... I was a bit worried and started to monitor more often ... it got worse so I traded it at 77,000 miles
as yours, no leaks visible, no smoke etc. but the oil pressure was lower than it had been and that worried me even more so I got the gauge checked and it was correct ... as I say I traded it for a VW for the wife because I didn't want to sell to a forum member and I'd do exactly the same again
the bright side of my story is that my next car was a DB9 so I did OK on the deal