Well, you need to go ahead and remove the spark plugs to see if the engine is seized, or perhaps the carburetor has developed a leak and filled the cylinders with fuel, producing a hydrolocked condition.
Another thought I have on the situation concerns the blown fuse.
The fuse should not blow just because you ran the starter for an extended time. The fuse should only blow if a shorted condition develops in some circuit that is common to the fuse.
I haven't known of any starter solenoid circuits having a dedicated fuse just for the solenoid control circuit. Usually, there is just a fuse in the circuit feeding input to the key switch and a short that develops anywhere down stream from the fuse will cause the fuse to fail.

How much oil did you pour into the engine? It sounds, to me, as if you filled the oil to the top of the dip-stick tube! If so----------Drain it all out, and only put enough oil in it up to the top cross line on the dip-stick. If you will inspect the cleaned-off dip stick, then insert it again and pull it out, that will show how much oil is in the engine. Peer at the bottom end of the stick very carefully-see the hash marks and cross lines. If the oil level, as it is right now, is way above the top cross line, you are in deep doo-doo. Do this before draining out the new oil!

Thanks for your replies,

the fuse that failed was the supply to the solenoid energising coil - you can see it in the diagram here on p20 (http://static.fatsoma.com/gts-pdfs/westwood/owners-99.pdf) marked F4,10A. It looked like the fuse had failed through overheating rather than a dead short, as the plastic body was pretty misshapen (and it was probably more than 20 years old). I think holding the solenoid on for the extended starts the engine seems to need runs this fuse at or near its maximum current for longer than intended - having changed the fuse, it hasn't blown again since.

The oil dipstick has two marks on it, an L with a line and an F with a line, seperated by about 10mm of cross-hatching. I filled the engine to the F mark and, now that it's been turned over a few times, the oil has dropped about 4mm below the F mark, which is what I'd expect. I had heard that some people think "full" means "to the top of the filler tube" - I guess the world needs these people to keep mechanics employed :)

Even though it's thick fog and pouring with rain outside, I'm going to pull the plugs now and see what's happing in the cylinders - I'm going out, and I may be some time...

Thanks for your help - any more thoughts are very welcome :)

8990TRAC

Well, I don't know what the answer was, but I got it started...pretty good, considering I was already looking on eBay for new engines!

After I put the starter motor back on, I took the plugs out and found they were clean and pretty much as new, which is good, as I only fitted them last week. I tried it on the starter again, but still no joy, so I turned it over on the nut at the centre of the fan with a socket and breaker bar and found it turned over with no problem at all... So, I ran the starter off a spare battery, connecting the +ve lead direct to the motor and after a bit of sparking and crackling, it turned the engine over (still no plugs in). So, to cut a long story short, I put everything back together again, turned the key and, after three or four 10 second attempts, it coughed into life - it's running at the moment and, with the new oil and filter, sounds a lot smoother than before!

I suspect a bad spot on the starter commutator and, through bad luck, managed to keep stopping the starter on it - pretty unlikely I know, but what else could the problem have been?

Anyway - I'd better go and stop it - thanks for your help etc,

8990TRAC

Well, you may be correct about "a bad spot on the commutator".
Typically, if a commutator ever develops a burnt place on the bars, that spot wears more than the rest of the commutator.
Once it has hollowed out some of the copper, that spot becomes a detent cavity for a brush to lodge into as the armature winds to a halt after you use the starter.
And because of the tendency for the armature to stop with a brush seated in the cavity, further burning continues.
Perhaps you will want to dismantle and inspect/service the commutator during an off season. Or you may try to find a "used" starter to work with and keep handy. I've a feeling you haven't heard the last of this.

In looking at the two diagrams offered in your link, first off........there are no Kohler engines featured in either diagram.
Diagram on page 20 states for Tecumseh and Briggs.
Diagram on page 21 states for Lombardini diesel.
The fuse you cited by number (F4 10amp) on page 20 is the feed circuit leading to the ignition switch. Fuse F2 6 amp actually does protect the solenoid control circuit.
Page 21 (Lombardini diesel) Fuse F3 10 amp is the feed to ignition switch while Fuse F2 10 amp protects the solenoid control circuit.

So.............do you have a Kohler? and the wrong manual?
Or have you mis-identified some other engine to be a kohler?