Engine is rebuilt after my, ahem, clever driving through a river plan

I think my starter motor is toast.

I’ve checked it my connecting it to a car battery with jump leads then used a power supply to the solenoid. The solenoid clicks and the toothed wheel tries to
Move but doesn’t.

****ed right? Or have I not set it up right?

Connect the jumper to the main feed to the starter,missing out the solinoid this should turn the starter. If that works it’s the solinoid that need pulling apart & freeing up.

Chances are something has seized together after the swim.

Jumper earth to mounting flange, jumper positive to the solenoid nut furthest away from the starter body. Spade terminal with a length of wire to the spade on the solenoid. Touch that wire to the positive jump lead clamp to spin the starter.
No spin? Connect the positive jump lead to the nut on the solenoid nearest the starter body. If starter now spins then the solenoid is faulty, these can easily be taken apart and you'll probably find the brass contacts are a bit manky and just need cleaning up. If it doesn't spin then take off the cap on the rear end of the starter to expose the brushes, these get stuck in the holders and the finger springs can't push them into contact with the copper armature. The armature can be polished up with a bit of fine sandpaper, free off the brushes and it'll work again probably.

Excellent knowledge as usual. Thanks