or it could have corroded, though if its aluminium that doesn't seem all that likely. .

In a UK winter with lots of salt unprotected aluminium will corrode pretty quickly. But I doubt it would in a 7 years old (max) car.

But read the first post. The OP says it was simply loose thus its still there and no mention of corrosion.

Without seeing it we can only guess the issue but back in my days of Triumph Spitfires it was a standard mod to remove the front backplates. With it in place the fronts would overheat in normal road use and going down a long hill in busy traffic in the Peak District the smell of burning is not a great idea. Don't ask how I found out.

I have nipped out to look at my Golf and I agree these are aluminium. I have never used one of these “brokering” firms to source a repair but maybe I am lucky in having access to a couple of good local indies.

Phoning up one who has already quoted may work but, if possible, I would find a recommended independent garage and ask them to look at it and give a price. For a job like this I would expect them to give a price just by looking at in in situ.

i just can’t see why the existing quotes are so high, unless garages don’t like these repair broker jobs and so try it on?

I suspect the OP may be looking a free bit of advertising for his company. I can find no reference to it on companies house and the independent reviews are a bit 'curate's egg'.