I've had a look in the manual for oil capacities, and the diesel has a much greater oil capacity at 5.9litres than the 1.8 petrol at 3.7litres (oil required when oil and filter are changed).
Using Halfords prices for Castrol Edge at £43 for 4 litres, I reckon the petrol needs £40 of oil, the diesel £64 worth.
(Another thread on which oil is here:
Which Oil in your 2.2 CTDi?)
I still can't see why the diesel is so much more expensive to service than the petrol. OK so there is £25 worth of oil, and the time it takes to pour in the extra 2 litres of oil (say 5 minutes at £60/hour equals £5 labour) to make £30 plus vat.
Next time I'm at the dealer I'll ask.