I'm capable enough to install the equipment.
Except that you do not know what to do.
but i will of coarse get a pro to check my work.
You can, but why not ask one first and he can tell you what you should do.
My own thoughts are to fit a fused spur next to the interior socket,
Yes.
You will need an RCD one if the circuit is not already protected by one if the new cable is to be concealed
and not sufficiently mechanically protected.
run t&e up and through the walle
Not through the cavity.
into an ip rated junction box,
Why do DIYers love junction boxes?
t&e to each of the exterior lights. t&e to an ip rated junction box to connect to pond pump.
Don't use t+e outside - use hi-tuf (google) or similar.
If this is fundamentally wrong , please let me know, because that is what I intended to do. My reason for asking the forum for help was to include a p.i.r. and some kind of over ride. I asked the question broadly so as to glean as much info as possible from the pros that do it all the time. Like maybe I need containment for the t&e and maybe I need a separate circuit from the consumer unit.
It is a simple job but quite involved.
From Fused Connection Unit (assuming the socket is not already a spur) run the cable to the switch and then to the pir and on to the first light then to the second then the third (no junction boxes) depending on the positions of the appliances and accessories.
Most pirs can be manipulated by switching to have the lights permanently on so the switch can also be used for permanently off.
Then there is testing with some expensive equipment.
That will cover the lights.
The pond pump could be a different matter.
An electrician on site will be better able to advise.