I have just completed an auto to manual conversion in my 74' TD 6Cyl Cortina.
I have installed a fully reconditioned (new bearings, snap rings, syncro rings, needle rollers, thrust plates, interlock collar, seals, gaskets etc... the works) BW single rail.
It's been filled withed 30 Grade oil.
Also I have installed a machined flywheel fitted with a complete new clutch (pressure plate, driven plate, throw out bearing).
Also fitted is a brand new clutch cable.

Everything has painstakingly been setup to the factory workshop manual specs.

On first fire up I tried reversing in the carport - All good !
I then drove the length of the street and back - All good !
Next I drove 2 km's - All good !
At this point all shifts were clean and sweet.

I then proceeded to drive the 9km's into town. by the time I got there, I was finding it a little difficult to engage 1st, 2nd & reverse gears.
I headed for home and it got harder to get all gears to engage.
When in any particular gear there is NO noise, whine, shudder or vibrations.
Stopping in front of my house to reverse up the driveway, I could not engage reverse at all. It would just grind when I attempted to push it into gear.
I turned the engine off, and the box slipped easily into reverse.
I restarted with the clutch in, backed into the carport and turned the engine off.
Trying the shift at this point (engine off) it slipped repeatedly, cleanly and easily into all gears.

Could the problem be, just the clutch and cable etc bedding in, or should I be looking for something more sinister?
Any help would be appreciated.

Try re-adjusting your clutch first & then see how it goes.

You didn't mention fitting a new Spigot bearing, or fitting one at all.....did you fit one?????

I should have mentioned that I have indeed fitted a new spigot bush, one of the original type that were fitted to GTHO's.
I have also fitted new universal joints to both front and rear of the tailshaft.
So mechanically, the car is brand new from the back of the engine to the front of the diff.

I have since re-adjusted the clutch cable, after finding it had gone from 3.5mm clearance to nearly 10mm, and after test driving it for approximately 45km's it has behaved properly. All shifts are grind free and slipping in fine. There was only one time it felt a little like it wasn't right into reverse gear, so I might give the reverse adjuster screw a small adjustment.

(at around 140Kph there is some vibration from the rear of the car, so I might look at getting the diff backlash checked. Otherwise the engine, box, clutch etc feel smooth and strong)

Cheers and Thanks.