[AndyV7o]

There is no such thing as a diesel oil really. Diesel oil just means it is high in detergents, but this level of detergent doesn't harm petrol engines. Good oil nowadays is for both engines and would've been termed 'diesel oil' a decade ago, essentially, 'petrol' oil has been done away with.
Have you looked on Opieoils.co.uk?
There's a forum discount too, iirc its Volvo10

Plus, if the specified grade is indeed 10w40, then 5w40 and 0w40 are both totally compatible and readily available. They are all essentially 40 grade oils, but 5 and 0 circulate faster/easier when cold. All three are the same at operating temp, which is around 1/6 the viscosity of 0w at 40°c, 1/7 the visc of 5w and about 1/9 the visc of 10w at 40°c if anybody decides to try and claim that 0w40 is 'too thin' 👍
If 5w30 then 5w30, 0w30 both absolutely fine.
Check what ACEA spec the manual requires not just the viscosity.