So with horn relay unplugged the car starts normal, and with it plugged in, the starter turns but doesnt engage.
Tells me you have atleast 2 skinny wires on the starter - either both push on terminals or one is bolted to the solenoid bolt which has the short length of wire that goes inside the starter itself.

How many wires are on your stater? and what colours are they? ( clean them good so you can see 100%)
What colour wires are on the horn and its relay?

the only wires to the starter are the positive one from the battery to the correct terminal as per the photo above and the black/yellow trace to the push on connector. that is all!
this is how it has been the whole time, with the push on one disconnecteted and the horn puggled in at the realy it wants to still spin the sarter as soon as the battery is hooked up.
toddat must have hooked the air horns incorectly as it only happens witht the realy connected up. dont know what colour the wires are as he has replced the horn wires.... alll in red!
all other eclecs work as they should... the usuall crappy datsun way.

but now its running ill drive it to the auto elecs closer to time of rego, just need to fix some minor surface rust and get some new tyres for reg... and $560 for reg in vic now. almost as much as the car cost me all up.

Ok so the horn wires are definately stuffed up.
But as to the starter. You say with the horn connected the starter turns but the engine doesnt.
And with the horn disconnected the starter works normal and the engine will turn over.
It would be good to know if with a good battery and the horn connected whether the starter turns over the engine or not.

with the horn connected it spins the starter as soon as you hook up the leads to the terminals but not cranking the engine over, just spins the starter.
disconnect the realy to the horm, it cranks when you turn the igniton key.
it did this with a fully charged battery but trying to work it all out drianed most off the battery.. will charge during the week and work it out over the long weekened.

There is no way a good starter can engage one minute and not the next. with only those wires.
Only way is if with the horn connected its making a really big short and this is sucking the power out of the battery. And if its that bad you should be seeing smoke and melted wires.
Its just not physically possible for this style of starter to do that.

There has got to be a power supply to the large solenoid terminal closest to the block to make this happen.

In normal operation, the high amperage current flows to the terminal furthest from the block [see photo], through the contacts in the solenoid, then back out via the terminal closest to the block, into the starter. The only time these solenoid contacts are touching is when the "pre engaged" pinion is pushed into engagement with the ring gear.
To just spin the starter alone means that this function is bypassed, so having a good look at the photo & comparing it with the actual wiring will reveal much.

I suspect that someone has used the wrong solenoid terminal for a power supply & power is flowing down this wire directly into the starter. Removing the relay breaks this circuit. Thats my best guess.

Unfamilia
How about a good quality picture of the starter,.. in situ, to help with diagnosis.

will take photo on friday or saturday and post it... will also trace the horn wires and also replace the sports steering wheel as it was not doing this with the sports one on, just when i replaced it with the stock 1200 one.
And its not the horn button stuck down as the starter wold not be doing this function with this problem
no somke or melted wiring either (checked under the dash... and bonnet.
I think it all leads up to this rear battery conversion from the last owner.

well no more trouble with coupe, started, jus need to set the idle.. had to go home.
the clutch is adjsuted and also the cooling system bled for bubbles. took it for a drive for about 2kms... the exhaust is loud but the thing goes well and feels good.
just have to fix theminor rust and tidy up for rego.

anyone in melbourne have a set of seat belts spare for a coupe? will try rare spares too.

also the horn problem has been resolved, just need to re wire it now, the positive for the horn compressor was wired directly to the main power feed, as it was not earthed properly, shorted the start circuit.

thanks for all the help to those who contributed.

I am glad that you were able to get it home so now its in a more suitable place for you to work on it.
As to the horn - it doesnt make sense - but as long as you can repair it --->
Nothings explained the starter not turning the engine over yet unless it has an extra wire - but as long as it works now ----->

the horn problem was that the power for the horn realy was connected to the main power feed to the fuse box, as the horn was not earthed correctly, it shorted the whole system, just have to hook the air horn power to the accessory power feed.
will still get an auto elec to check it out closer to rego date.