Aqualisa Shower leaking from pump/mixer unit?

My Aqualisa shower is leaking from the mixer unit. I took it appart, down to the iternal pump. It seems that when it is turned off in full, there is quite a fast leak from the front of the pump unit. I loosened it a little until i found a point at which the leak through the pump stops but this means the shower is not turnerd off fully and the water constantly drips/trickles through the shower head. Being a single mum at xmas, the last thing I need is call out charges engineers etc. Would anyone have any experience of fixing this problem themselves? Thanks. Happy xmas