Aqualisa Aquavalve 605 is leaking....

I have an Aqualisa Aquavalve 605 in the house we have just bought. Th
unit is outside of the warranty period and has developed a leak fro the back of the unit. The water is not coming from any of the pipe jointed into the back (i.e. the body) of the unit, it is instead comin from the centre of the unit itself.
Any ideas what may have gone awry here? The construction suggests to m that it may be the "Gasket with filter" which I think provides a seal t the cartridge. However I have read a number of posts which suggest i could also be the cartridge. Any ideas which it is likely to be o "tests" which would indicate which part was at fault.
Any help anyone can give would be very much appreciated. As you ca probably tell I am clearly a novice!
-- GazzaT
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