Sorry I misread A55ii's reply. I don't want to remove the screw to measure it as I have fairly new brake fluid in the system and by the time I've measured it, all the fluid would have come out and I fear it would get contaminated and unuseable.
I just measured the exposed part of the screw and its 15mm from the bottom of the nut to the top and 5mm from the top of the nut to the top of the screw.
Sorry I misread A55ii's reply. I don't want to remove the screw to measure it as I have fairly new brake fluid in the system and by the time I've measured it, all the fluid would have come out and I fear it would get contaminated and unuseable.
I just measured the exposed part of the screw and its 15mm from the bottom of the nut to the top and 5mm from the top of the nut to the top of the screw.
as said before, now you have it in your hand, take it to your local motor factor who will be able to match it exactly. They can also sell you the correct spanner / wrench to use so you don't damage another one .