Worcester 'bathroom' tap

The sink tap in our shower room has become loose.  All I can see underneath is a 'grub' type screw which I thought probably needed an Allen key, but seemingly requires a plain screwdriver.  Before I start messing about with it is there anything I need to know about how this tap is supposed to be fixed, please?

Hi Hblewett.
I have the same problem at the mo with our sink tap. I removed the back plate in the swinging wall shower unit and thought I had sorted it, using a screw driver to tighten the bolt under the tap unit. I then tried to tighten the nut on the bolt to secure the tap unit tightly to the underneath of the sink.
Sorry to say I failed to tighten it enough to fully stop the tap moving. 
So like you I shall wait for any further remedies on this, our useful info forum.

Best wishes  Eddie

Thanks for your reply Eddie. I should get a chance to have another look at mine tomorrow. If I achieve anything worthwhile I’ll let you know!

I got this fixed in the end - it's a straightforward tightenening up of the nut, just difficult to see without a good torch and an uncomfortable upside-down position to see and get at it!