I f*cking hate indoor interval training!

You saddle angle or saddle is off if your nob is going numb.
I've been there, it's not fun.

Maybe it's level when both wheels are on the floor but not when strapped into the turbo, check, make sure it's level when on the turbo, or slightly down but only a tiny bit.
Also, do you stand up at all to relieve pressure? I stand every 5 mins or so for 5-10 seconds to relieve pressure. Also helps with reducing the likelihood of sores developing.

Little update:
I've been using some fairly cheap own brand padded shorts I bought from Condor. These were still in the wash so I used a pair of bib shorts last night. Result - no numbness.
I bought the shorts specifically for wearing under cycling trousers which I only wear when riding my steamroller. Their purchase also coincided with fitting a new saddle and a lot of discomfort despite having the same saddle on another bike with no issues whatsoever.
Looks like it's the shorts that are the problem...