Ok, back in the game! What a ball ache that was. Found this: https://www.reddit.com/r/Surface/comments/gxgjfz/surface_book_3_cpu_stuck_at_400mhz/?utm_source=amp&utm_medium=&utm_content=post_body
To “solve” it I detached the screen and pressed the top left button and the + to shutdown and the rebooted it. Bingo, CPU now clocking happily.
So now i just have setup the laptop once again grrrr.
hope it helps