Cannondale CAAD Optimo Tiagra 2020 CREAKING I am a newbie when it comes to bikes and I recently purchased a brand new Cannondale CAAD Optimo Tiagra 2020. After about 3 small rides (no more than 5 km a time), I noticed a very loud creak start to appear. I figured it was just settling noise and maybe needed lubricant in areas etc. But the noise continued to get much much worse as I started using it for my commute (approx 2 weeks at 15 km a day).
I took it in for its 6 week service (at Evans Cycles in London) and was told (to my horror) that I need to return the bike to Cannondale as there is a welding fault in the area north of the front part of the frame that is causing the creak. However, when riding it, I can clearly hear the creak is coming from the crank area and appears to only creak when pedaling, not even when cruising (which you would expect if it was coming from the front of the frame?)
I am really hesitant to return the bike as they said the lead time is 6 weeks and I am not guaranteed the same bike I have bought. Just the next best equivalent. Which in itself seems bonkers. I am using the bike to commute in and out of London so I really don't want to go 6 weeks without it. For cost and obvious health reasons.
I guess my main questions are, fundamentally, is the bike safe to ride? and do you think I should get a second opinion on the source of the noise, problem before admitting defeat and returning the bike to Cannondale for them to check? I have read a lot of forums and it looks like Cannondale have this issue a lot.

Mod note: This question is about a brand new bike frame that creaks, with a frame fault, and an LBS instructing to return it for repair.  This question is not about bottom brackets that creak.
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.