So for anyone in the future who wants to know how to do this I will post a summary of what I did.
Problem - steering seized as F
I disconected the steering arm - motor swung easily so it wasn't the bearings or the bushings in the leg of the motor.
I next disconnected the cable from the steering wheel - that turned easily so not the steering wheel or box.
Then I had to get the cable rod out of the tilt tube.
This was not possible because of a dip in the transom not allowing the rod to be pulled out. I undid 3 of the 4 bolts holding the engine off (had to angle grind the engine bolt lock off too as also seized).
Anyway with one top bolt holding the engine on I could tilt the engine sideways (and therefore upards on one side) in order to get a wooden shim under the bracket to hold the tilt tube up out of the way so that the steering rod could be removed. Obviously you have to undo the steering cable lock nut first.
Anyway the steering cable rod came out easily...it is rusty in the tube and I will clean it out with spiral wire brush but the rod moved very easy and so that was not the problem either.
Leaving the steering cable as the broken item....
I attached using cable ties and a jubilee clip for good measure 2 4mm cords to the steering end of the cable and pulled the duff cable out carefully. It was a bit of a fiddle and helped by someone at the other end feeding it in as I pulled and also one tiny but convenient inspection hatch that I could just get a hand in to help it all along.
Once the cable was out it was clear that it was literally seized rigid so clearly that was the root of the trouble.
Reassembly with the new cable (£150) was the reverse process. Pulled it through on one of the cords again with cable ties clamping the cord leaving one spare cord in case of disasters. Pulled it through from stern to bow.
Jobs a 'good-un'
When I can get an engine hoist on it I will do the tilt tube over the winter and am looking forward to coaxing that baby out of there - presumably I will have to get some heat on it or it won't play. You never know it might just slide right out..... Yeah right
Anyone got any questions am happy to help.