maybe reiterate what happens ? that the RCD does indeed trip taking out other appliances when you try to switch the lighting circuit back on.
if (tomorrow) you turn off all the cicruits , does the lighting circuit alone then work ?
so as Bigboy suggested you may have another circuit that is leaking charge into the lighting circuit when it is switched on as well,
so turn the others on and off individually until you find it.
my suspect, like I mentioned might be the one with the UPS - if you have now permanently turned off the immersion switch it can't be that.
as you say, it all sounds weird that you have done nothing to reconfigure the appliance connections in the house and the problem has started unprovoked, maybe when you had turned of ups circuit earlier it started having a problem
e:
I agree it's not clear
Nope, The RCD isnt tripping, the lights are not even linked to an RCD from what I can tell.
The RCD is green, and seems to be linked to my sockets and shower.
Then all the red switches are lights, lights, immersion and oven then at the end is a red master switch.
The switch thats tripping is the first lights switch, RCD stays on, everything else stays on.
If the mains is off, the lights switch still trips, it has all the ballparks of a failed circuit breaker/switch.
I can post a photo but the labels wont be readable as the angle and distance to the box doesnt get proper focus.
I can try the new test you have suggested of which I try to only turn on the lights switch by itself.
I am not sure though why you think a UPS is tripping a lights circuit on a different circuit and this problem would start simply from turning the lights switch off then on again.
--
Done the test
This will probably be the last one as I dont want to keep switching everything on and off now.
I turned off all switches gradually, then with all of them off including the master tried to turn on the first lights, it wouldnt stay on.
I then turned on the master and tried to turn on the lights by itself with the master on, wouldnt stay on.
Have now turned the rest back on.
Also the lights switch that wont turn on always clicks when I push it up, whilst the others dont.