In article ,
Chris Bacon writes:

There will be four, 2x5' slim tube ones,

Tubes are 58W, assume 65W with control gear losses,
which is 520W total. This is going to be about 2.5A,
or 5A if they are cheap fittings with no power factor
correction capacitors.

1x4' slim tube one, and

Let's guess 45W with control gear losses, i.e. 0.2A,
or 0.4A is no power factor correction capacitor.

an ancient "fast start" one with a fat 4' tube and a nice polished
aluminium reflector.

Probably about the same again (not sure what type of control
gear you are referring to).

So worse case, it's 6A.

Cable is 1mm in the garage, fed from a 2.5mm
feed from the house to a junction box, via a high(ish) level 4m
galvanised conduit.

--
Andrew Gabriel