How to replace a clockwork timer on a Magimix food steamer?

Magimix food steamer model no 11191 has a faulty clockwork timer. Have new timer but cannot remove steamer base to gain access for replacement.
Base fixing screws are of the type with a 6 pointed star shaped indentation in the counter sunk type screw head. I have a suitable tool but there is a raised pin at the centre of the indentation preventing use of the tool.
Can any one help.Is there a very particular tool that I need?