Check out the 20k BMW, Audi, Merc oil changes on YouTube - black sludge. I don't believe that oil is meant to last that long. I had my oil tested at a lab (out of interest) after the first 10k in my Audi a few years back and they said it needed changing, although the computer said it could run for another 9k.
some good advice above although still wanting to understand more about the "essential" servicing for older cars (older than 5 years) which would bring the cost down quite a bit - does this include spark plugs, air filters and other expensive items on the 40/60/80k standard services - Id imagine they have to, therefore much better value at £165 for interim and £345 for major services - personally I don't mind paying that. In fact the major service could be a real bargain for the 60k/120k where the plugs need to be changed.
Will probably take it to an independent garage for brakes etc though.