This is one of the great undiluted joys of being a bike mechanic, which I’m not by trade but am by about 28 years practice.
My Thorn Raven developed clunkiness in Bottom Bracket so I deduced that it needed to be replaced. It is in an eccentric bottom bracket shell as it has Rohloff gears. I replaced it and clunkiness disappeared but in first spin out I noticed a creak from the bottom bracket area. Now there are legion causes of this, so I stood up. It persisted. Not the saddle or seatpin then. I noticed that it happened only with pressure on the left pedal.
Came home. Tightened pedal. No good. Tightened crank. No good. Took off pedal and crank, greased them, refitted. No good. Pedal was fairly new, so I put another one in. No good. Tightened eccentric screws which keep eccentric in position. No good.
Maybe it’s the crank, as I’d had a little trouble with a crankpuller when replacing Bottom Bracket and slightly damaged it. Took crank off old bike and swapped. No good.
There’s only one other thing that I can think of, but I did tighten them recently. Chainring bolts. Ah ha!!. Two were slightly loose. Eureka. Fixed. The creak is gone and I am not up the creek!
It only took me about 2 hours..
The lesson must be this, something I didn’t know before. A left pressure creak, think of chainring bolts.