Wow just catching up on this seems a bit of an iffy one.
One thing I noticed with who is responsible - basically in the UK if something is faulty (in or out of warranty) in a reasonable time period it starts with the retailer who can them claim from the manufacturer to the point where the manufacturer could claim off the material supplier, in this case the silicone etc. The point is its meant to be as easy as possible for the consumer without having the back and forth you have going on here. You shouldn't need to contact Juwel direct the retailer should basically accept there is a fault with the tank and it needs replacing, it is slightly more complicated in this case because you have the welfare of the fish to consider.
Have you dropped the bomb of talking about trading standards? That usually gets things moving - check out Martin Lewis's guides on how to return things too. You are 100% in the right on all of this so just stick to it
If you still wanted to go down the route of a new tank, I had an ND aquatics tank for just under 10 years and when I sold it, it was still as solid as day 1. The other manufacturers to check out are aquariums 4 life and Wharf Aquatics. Give them all a ring though as they are all pretty old school and dont really reply on emails - I think they are all also pretty busy so if emails are not top priority for them they will slip to the bottom of the list. Also all worth a ring to see if they have anything in stock - eg when I got my tank from ND they had a few different sizes they just make for stock and then when the order comes in just make the stand which can make a little cheaper - only issue is they are usually 18 inches front to back which isnt great for bigger fish.
Wills