Sorry - I should have said that it was a maximum potential load
of 600W. It actually only has 300W at the moment, but some
of the fittings can take higher capacity bulbs, so I ought to have
a setup that allows for that possibility. These are external lights,
and are spread far and wide about my property, so that I can
see where I'm walking at night.

300W is certainly better, although you should consider using CFL type bulbs
for this application, to reduce usage further.

Personally, I would wire this up differently from a traditional lighting
circuit. I would use a DP switched immersion timer (or any suitable light
switch and relay) and have independent RCD protection, too, run from a 3A
MCB on the consumer unit. This reduces the effect of faults in the garden
circuit on the rest of the system.

Christian.