I'd go for a proper repair if it was me I'd spend the £400+ on a new bumper as I wouldn't want filler on my week old car.

A bit of plastic filler won't be the end of the world - it's a plastic bumper after all anyway!!

Just to add another experience. I previously owned a Volvo and I don't know how, but a metal rod thrown up from the vehicle in front punctured a 1" hole in the bumper. A local bodyshop did a great job of filling and re-spraying. Still not visible 3 years later when car sold. Being a local independent they didn't charge the earth. They have all the gear and a good paint/drying booth which the mobile guys don't have. Most dealers farm jobs out in any case.