It's difficult to say with any certainty but the difference is only about one and a half inches in old money so I'd imagine that the awning would be ok on reasonably level pitches.
i would say try it if its 4 cm small thats only cms each side i alwats prefer a tight awning than one thats loose and flaps around and the roof filling with water saves spending any way.
Thanks for the replies, I spoke to Isabella and they think it will be a good fit as according to them a 875cm awning will fit vans with an awning size of 865cm - 885cm so I have now purchased the awning. Thanks again.