cldlr76: Cool thanks for you help.
The 2 black cables come into the house from the demarc point on the outside of the house so there is probably only about 500mm of that cable so I'll just remove the electrical connector block remove the white cable and use the gel connectors to join the black cable to the splitter then run cat5 to the 2 jacks.
Does the cupboard have a space where you could permanently (and tidy!!) fix the splitter without being obtrusive or touching stuff in the cupboard? then you could easily connect the master jack to the filtered pair and another jack to the ADSL pair.
Where are you planning to put the modem, nearby or pretty far away? I have $5 worth of Cat5 that would do it if you want some help, but you also want to tidy up the stuff in the garage though. Maybe could install a punchdown block high up on the wall there.