Thanks Michael and Dan for your prompt reactions!

Michael, your first method is a bit fuzzy to me, but don't bother explaining it otherwise, the second one is working for me, and I am glad I learned about the construction line trick!
Dan, your method is good, but also rotates the mirror along the Y axis, which is not desirable in this case.

but when reading all this, I suddenly realised I was focusing so much on the rotation aspect, that I forgot the orient tool, which beats the above solutions! simply select the plane and the normal line, select the orient tool and adjust base and target orientation. no additional construction lines to be made!

thanks, I wouldn't have found it without your solutions, and I learned a few things more about MoI!!!

rgds
wim