Oil, like most fluids we put in our cars, starts to break down from the second it goes it.
Up until about 5 years ago there was no oil manufacturer who would guarantee certain properties of their product would still be met after 7000 miles in a fairly high performance, though NA, engine. Given that info, I would not consider sticking to manufacturers schedules, that are not their to prolong the life of the engine, they are there to be the absolute minimum to get the car through its warranty period.
In a turbocharged car, which all diesels are now, and more and more small petrols are, you are destroying the oil faster.
The Independents, as has been stated, won the argument about non dealer servicing years ago. DIY follows the same route, it would be up to the manufacturer to prove that it was an error in the servicing AND that you were not competent. The grey areas might be if a non genuine part that was fitted failed, as the car manufacturer could claim reasonable diagnostic costs.
As said previously, if ultra concerned about the arguments over warranty, take it to a Volvo dealer for an oil change only, though will probably cost £100-150.