Dripping bathroom tap

Mollieperkins

Hi we have a dripping bathroom tap in our 2011 Bailey Unicorn Seville and are unsure have to fix it.

The tap is square, my husband has taken the handle off to try and repaire it but there is nowhere to get a spanner in to unscrew the cartridge.

Does anyone know if there is a special tool needed, Or has anyone fixed one before? 

He has fixed many taps but this one has him stumped 🤔