Cannondale CAAD Optimo Tiagra 2020 CREAKING

return the bike to Cannondale as there is a welding fault in the area north of the front part of the frame

If true, that bike is probably not safe to ride. If the shop showed you where the problem is, posting a picture of that area would probably be useful. For what it's worth, if the shop that sold you the bicycle says it has an unsafe frame, I'd tend to believe them as that's an admission of a serious failure on their part, especially if they assembled the bike for you.

If the frame is faulty, Cannondale should warranty it no problem.

I can clearly hear the creak is coming from the crank area and appears to only creak when pedalling

That does not rule out frame issue causing the creaking sound.

First, it is extremely hard to isolate the actual location on a bike where a sound is coming. And even then, because the components of the bike and and do conduct sound, where the sound appears to be coming from might not actually be the source of the sound.

Second, pedaling puts stresses on the bicycle that don't exist when you're just coasting. Since significant power is applied to one pedal at a time, and the pedals are off-center, pedaling introduces twisting and sideways forces to pretty much the entire bicycle. So a noise that is synchronized to your pedaling could be caused by something like a cracked front rim.

Finally, and completely independent of any frame weld issues, if the bike has a press-fit bottom bracket*, those are notorious for being noisy and a constant source of creaks and cracking noises. This is a Google search for all the questions here about noisy press-fit bottom brackets: https://www.google.com/search?q=press+fit+bottom+bracket+noise+site%3Astackexchange.com

* - It likely does, but I can't confirm that from the product page as there doesn't seem to be a 2020 page but only a 2019 page that doesn't actually specify the bottom bracket type.

As others have said the bike may not be safe to ride. A frame failure when riding can cause a nasty crash. Don;t ride it.

You must return the bike. It's faulty and dangerous. I think there are some issues with what Evans cycles have told you to do though. I think you are saying they told you to work with Canondale directly to return the bike and get a replacement. That does not seem right. In your situation upon being told the bike was faulty I would have demanded the shop take it back and refund my money, immediately. Evans, as a Cannondale dealer, can then deal with the problem. If you have you money back you can immediately look for another bike to purchase rather than waiting for Cannondale's replacement process.

You are right that not being offered an identical replacement is 'bonkers'. However, I strongly suspect the massive increase in demand for bicycles during the COVID pandemic is what is causing this. Cannondale just does not have an identical bike to offer you. This is another reason to demand Evans take the bike back and refund you.

Good luck finding a replacement.

My 1990 Cannondale had a creaky bottom bracket that the dealer could not fix. Crazy that this is still a thing. I was only able to address it by completely disassembling, cleaning, and reinstalling with LocTite High Strength locking adhesive. Luckily I've not had to service the bottom bracket in the subsequent 30 years.

I have the 2019 version of this bike, also bought from Evans. The bottom bracket is threaded (a screw fit) and unlikely to be creaking already. I do agree with the other answers on safety - the frame is safety critical and you could cost yourself more in dentistry than a bike costs.

Evans' liability is only really to refund your money. Bikes are in short supply, but they are a big outfit and should be able to find something even if not another CAAD Optimo. They seem to have Cube Attain and Specialized Allez in stock; these both have geometry that is a bit less stretched out and racy than your Cannondale but are still decent bikes. So if you're offered one as a replacement, you might do worse. Otherwise find something that is in stock and insist on a refund. You can get some good stuff for about £1000 and I'm guessing you paid £900 ish.

Mine is silent apart from some chain rub on the front derailleur when pedalling really hard, and only then on the drive side. If your creak is really from the crank check that the cranks are tight. I think it's an 8 mm hex key to tighten on the non-drive side, but with the noise you describe, you'd likely be able to feel any looseness. But please do heed the safety warning.