At £6, you can afford to give it a whirl - it might be utter crap, but if you take a picture of a white card first, then you can balance out the colour cast in PP, by using it to set your white point.
Alternatively, pony up the extra, and grab a Hitech one from eBay for £21 delivered, from Crooked Image, iirc.
Chances are the one on Amazon is the same £1.60 ones you'll see with free delivery from a China-based seller.
If you can wait, you'll waste even less money, but the consensus is these filters are quite poor.
Even if it cost £1 to return the Amazon one, you're still better off by 60p, and get to assess it sooner, since you'll throw the China direct one in the bin, if it was absolutely dire.
So with nothing to show for the amount spent, £1 is minimal risk to gauge whether it's useable or not.