C3 is for VW cars so it should do you, did you check castrols oil finder app to see if it reccomends that oil? That would be a quick way to check
 
Ahh yes it's 504, think it's best to just go in to europarts and get some decent oil?
 
i've never used ECP for oil so I can't comment on their brands/prices.

I either haggle a local VW/SEAT/Skoda dealer down to £30 for 5l of Quantum longlife 3 or watch Halfords/Asda/Tesco for oil offers. Asda had an offer on 504/507 spec Castrol Edge at the start of the year for example. £30 for 4l.
 
I've just looked on euro car parts, they have castrol for £34 and a filter for £7... Seems cheap to me