Looks like your course of action is :
- Speak to the dealer again, and press your point.
- Talk to Volvo for their view.
- ]Finally, if no luck with either get an independent expert's view who will hopefully back your view point, which may ultimately involve a visit to small claims court though.
Be sure to let us know how you get on, as my Volvo has had 12 services, and would be interested to know if Volvo think I should have to account for a new sump in the near future.