Going back a few years the saftey switch (RCD), was usually only placed on GPO power circuits, if this is the case at your house, then should not be your HWS or lighting. Usual suspect is a fridge/freezer as you know, next in line in my experience are GPO's with moisture getting in, any outdoor power points? Also have found power points on the internal walls of houses with 'moist crud' sitting on top of terminals - any power points under a window? Intermittent tripping like yours is the hard to track down without an Insulation Tester, you may have to call a sparkie.

Cheers
Dave