Tightening Mixer Tap

Hello

The mixer tap on the small basin in the washroom in our Symbol Plus has become loose after just 18 months. It's not at all obvious to me how to tighten it, especially since there doesn't seem to be any straightforward way to gain access to the underside of it given the closed construction of the plastic basin. 
Does anyone have any suggestions? 

Many thanks in advance.

Does your sink look like this?

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There’s a similar thread in ‘Technical’ but for a Cotswold. Bathroom sink on Kingham very much like photo, isn’t sink stuck down? Looks like time of year for loose bathroom taps?

Roger That
Yes, it looks exactly like that. Can't see how to gain access to underside of tap without removing basin.
Thanks.