I'd use a better oil than Castrol really.
20% off that 5W30 castrol edge in halfords at the moment making it just over £30,
still cheaper and easier to get national to do it including the filter i guess,nothing against national and i have used then for tracking and tyre changes before but i prefer too service my own car.
Just had a look at the website and its £45.99 for 4l of Castrol Edge (Ibiza needs 4.5l). Is it only an instore thing?