I see. So you are in UK, you know Neil, and your pond has a viewing window. Now we are getting somewhere... You have just quadrupled your electronic footprint on the net
The fish look great!
I suppose I was duped in a similar way once - about several years back I saw a fish for sale on Ebay. The ad stated maculatus and it looked close, so I got it. $30. Its appearance was just like you describe of yours. But it grew up to look for all intents and purposes like a blochii, just with a better, crisper spot definition, other than that, I fail to see any differences.
You can spot it here in my blochii thread, especially in the latest videos:
https://www.monsterfishkeepers.com/...odus-blochii-in-4500-gal.682528/#post-7680734
I never searched intently but also I have never seen Pimelodus maculatus for sale at any LFS or on any mail-order vendor list that I came across in the US for the last decade.