Many thanks to all who have replied to this thread. I realise
from some of the replies that my description of what I want to
achieve could have been clearer so I'm replying to my own
original message.

First of all, these are not security lights and there was never
any requirement for the high wattage halogen type things - I just
want a bit of light so that I can take something out the back
to the bin at night without stepping in dog poo (our dog has
access to the outside via a flap on the back door).

Secondly, I had never really considered using PIRs - I imagined
having a switch next to the back door, though the replies I have
received have given me cause to reconsider this. Even so, I want
it to be possible to switch the light on or off (overriding
any PIR) from somewhere in the kitchen, even if it is not as
convenient as being a switch next to the door.
This, together with the requirement that there be minimal internal
disruption is what led me to the conclusion that most of the runs
of wiring need to be on the outside and why the approach that was
suggested of dropping the wiring down from the eaves would not work
- there is no easy way internally to get cable from the loft to
the kitchen.

Perhaps I should explain my original intention a bit better. The
door is on the side of the house. The understairs cupboard ('pantry')
is accessed from the kitchen and is where the consumer unit is
located. One of the walls of this cupboard is part of the side
wall of the house. What I had therefore imagined doing was to
effectively add a couple of lights to the downstairs light circuit,
with the cables on the outside of the house, only entering for
the switch next to the back door and into the pantry to connect
into the circuit.

This would have entailed a couple of vertical runs of cable that
would enter the house at a relatively low level. It is these that
I was particularly concerned might get knocked occasionally. (The
sort of situation I have in mind is where I am carrying an armful
of prunings from the back garden down the side path to the front
of the house. My experience is that they tend to scrape along the
wall, and catch on obstacles, though the force they exert is not
immense.)

If I go with the PIR idea and forget about the need for a convenient
switch next to the back door, I can get this down to one vertical run
that enters at the top of the pantry (about 5 feet off the ground).
I can put a slightly less convenient switch in the pantry for
overriding when I need it, but rely on the PIRs for day-to-day
convenience. Also, I can run this vertical cable behind the soil
pipe, so it should be pretty well protected.

In the light of the helpful replies (thanks to John in particular for
the screwfix details), I will use heavy duty PVC trunking. Is the
screwfix stuff definitely suitable for exterior use? Also, it was
suggested to use standard cable inside trunking - is this OK? As
far as I can see, only the temperature aspect of the weather will
have a bearing.

I'd like to take on board the comments about light pollution
from the bulkhead style lights. However, I would re-iterate that
these lights are to go on the side of the house. Also, next door
is about 7 ft away, so the light is not going to get much chance to
'escape', and next door already have a light of this type. They will
only be switched on for short periods anyway. Having said this,
I'll look into the "light in a box" that Jon recommended, and the
units that Andy recommended.

Thanks once again for all the help - brilliant response as usual.

-
Ian