Question about replacing a washer in a bath/mixer tap

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Hi,
I have a bath/shower mixer unit (Helix bath shower/mixer tap), which I had fitted a year ago.
Recently, when the water is diverted to the tap, there is a small leak coming out of the shower head.
It is the type of unit that you push a lever left to divert the water into the sprout (Swan neck) and right when you want to divert water to the shower.
I contacted the company I purchased the unit from and asked their advice and they advised that I should replace the diverter washer, but I cannot see how to access it.
If this is correct to solved the leak, any advice on how to replace the diverter washer would very much appreciated.