Paul,
An OEM means one made either by or specifically for Mercedes-Benz. If you can get a new OEM grill surround for 300 quid, run don't walk to buy it. From MB here in the USA last I heard they were in the vicinity of $2,200 each.
Fit is ALWAYS an issue particularly if body work has been done on the front. Be wary of stress cracks in the upper corners.
I exchanged from one vendor 4 different surrounds before we found one that fit properly.