You should get around 45mpg from a td.
Oil/filter every 12,000 miles, I would change the FUEL filter every 12,000 as well, for a LUCAS CAV one, tappets onl;y need doping about every 40,000 as shim and hardly wear.
As to the keys, the black keys SHOULD start the car and be the normal ones you use, the red key is used to CODE the black keys, so that they will do this.
Either the black keys have not been coded(using the red key) or they are duff.
tabetha