Good grief why are you involving the central heating connections? I dont know how to make this any simplier,
The brown wire is common to cylinder stat, cut it between the junction box and the cylinder stat,
connect the brown from the junction box to com on new switch
connect the other end of the brown you just cut to normally closed on the new switch
connect a new cable from normally open on new switch to 2 (the cable that is sleeved ) on the cyl stat block connection.(which is the hot water off connection)
The brown wire is common to cylinder stat, cut it between the junction box and the cylinder stat,
connect the brown from the junction box to com on new switch
connect the other end of the brown you just cut to normally closed on the new switch
connect a new cable from normally open on new switch to 2 (the cable that is sleeved ) on the cyl stat block connection.(which is the hot water off connection)