Hello Clemens,
Judging by Nick's post the 408 is exactly the same as the 411 in this area - Bristol didn't like to change things if it wasn't necessary!
I posted a picture of the hooks on the rear of the seat back in this post
rear seat 603
If the seat hasn't been removed for a long time I found it can take quite a lot of effort to slid it upward so the hooks disengage with the rear bulkhead.
Before you remove the sender unit I suggest you check the wiring.
Please let us know how you get on.
Kind regards,
Kevin