Another update. The telescopic steering shaft was made from square box section tube, welded to the U/J which in turn plugged into the lower sprocket on the trailer. The inner sliding part was a length of 1/2" square bar filed to a nice sliding fit. This was drilled and threaded then screwed tightly onto the round rod which formed the other half of the shaft. A pin through the square bar and rod made sure nothing moved. A short extra piece was added later to give a bit more bearing surface although probably not needed.
A bit of surface rust showing since it's last use. The large threaded hole is a left over from it's last life. The second photo shows it in it's storage position, held in place with the throttle cable. The trailer is disconnected by removing the throttle lever from the steering yoke, one screw, pulling the coupling pin and moving the trailer back which slides the two shaft sections apart. Coupling all together is just as simple.
The throttle cable had to be extended to approx twice it's length. Think it was originally fitted to a lawn mower. Couldn't find anything suitable on the internet. The inner was 1mm. Most others found were 1.25 or 1.5mm. I could have opened up the cable slot in the carb piston but decided against this as after a bit of hunting found 1mm inner cable was available from China. A 5 metre length for £1 25 including postage. A 2 metre length of outer sleeve was found in the UK but couldn't find this to match the inner in China. Strange.
After careful measurement the inner was cut to length and a short piece of brass tube soldered on one end. The carb top, outer sleeve etc was assembled followed by the lever end nipple being soldered in place. After connecting to the trailer everything was found to operate smoothly and with approx the same amount of movement as the original cable. This photo shows the new inner and brass tube nipple alongside the old.