Hi Extopia,
I am not an electrician. However, it might be best, before you start rooting around in the fusebox endangering lives, not least your own(!), to "isolate" the problem, if you suspect the garden lights then stop using them for a few days to see if your suspicion is right. Perhaps, (and I want to emphasise that I'm no spark), the lights were wired on the same loop as the house internal lights and, perhaps, the wire used was the wrong "thickness" and this wire is, hence, as the garden lights could be a large load, (how many watts each?), overheating and this heat is interfering with the internal wires?
I'd definately get a qualified spark out to investigate.
ALERT.