Creaking when in the saddle
Creaking when only in the saddle - I would check the saddle? A drop of oil into the where the saddle rails fix to the seat can alleviate.
But if you are sure it is not this - then perhaps try isolating the noise by using the bike on a static trainer and pedalling. If you remove the chain from engaging with the chainset - it would isolate the noise to the chainset / BB only.