They are not that rare but are only around for few weeks of the year.They can be up to 4 years as a grub living on rotten wood. Then when they hatch in June they are only around for 2 to 4 weeks.
They are quite big, up to 20 mm, I would say this was about 15 mm long. Also the pattern of the dark brown /black marks on their backs does vary quite a lot. Sometimes more banded other times more spotted. This one is sort of intermediate.
I notice there is a small drop of liquid on the leaf just behind the back end of the beetle. It had not been raining so I wonder if this is beetle juice/wee.