If you undo the four screws on the tacho housing and pull it away from the body of the boat, you will see all the wires connecting the tacho.
On the lagoon
380, there is an orange wire - this is the one which drives the hours clock. Cut the orange wire and connect a new wire to this orange one with a block connector ( but dont disconnect it from the tacho )Run this to the +ve side of the new clock and run another wire from the -ve side of the new clock to a black earth wire - also behind the tacho casing.
In this way, the new clock works, but the old one will still come on sometimes as it already does.
Not sure what make the new clocks are as the previous owner left them for me when he sold up.
Hope this helps !!