Filling a hole next to basin tap

Fogle1980

Hi, we’ve just had a bathroom basin tap (square in shape) replaced. We have a custom built tiled shelf which the tap sits on and freestanding sink.

The tap size at the base is 5mm smaller than the old tap and now fitted there’s a 5mm gap at the back of the tap. 

What would you recommend to fill the hole? Sealant or should I fill the hole with something first and then apply sealant? 

The shelf is quite tall so accessing underneath the shelf isn’t possible. Thanks 

Spies

Just white silicone sealent would do it

Eldi_Dos

Would a escutcheon plate give a better finish.