1995 Symphony toilet fold up sink unit

Water doesn't stop when tap turned off. There is a 'click' when tap turned on and water flows via the pump ok. It also 'clicks' when turned off, but suddenly the water now continues to trickle out of the small mixer spout.  Can anyone advise me on how to rectify this?  Can I remove these taps and replace a washer or something?  Any advise would be appreciated before I try removing the unit or doing something silly! Thanks.
Derek

Access to the rear of the sink is by removal of the rear panel by the back door.

I assume this is the standard mixer tap fitted by AS, in which case it is a Dimatec (motorhome) tap available from Magnum Motorhomes.
If water dribbles from the spout, the solution is to replace the cartridge in the upper body which contains a cluster of three ring washers which control flow and temperature.
You can do this without removing the tap. There will be an access hole for a small screwdriver, usually hidden behind a small plastic insert on the top or the side of the on/off lever. Slacken the screw to release the lever and expose the brass inner fittings. An adjustable spanner now removes a ring and the cartridge will lift out. It is necessary to replace the cartridge. They cannot be repaired.
Support the tap when exerting pressure with the spanner as the plastic sink does not have the strength of stainless steel !!

A 1995 Symphony will probably have this type of tap. No cartridge, just an 'O' ring.

Whoops !

Thanks for the advice received. The mixer tap pictured is the same as the one on our kitchen sink. The bathroom one is a 'comet mixer with micro switch'. I took the back panel off to get to the taps and removed them no problem. I have ordered a new complete unit from magnum motorhomes.  So both of you contributed to me sorting the problem out. Thank you.  Derek