First gear sometimes blocked


Bit of an odd problem in my 1.4 AUA. On the odd occasion the first time you try to engage first gear it only goes half way, stopping at the first "notch". It feels like it's blocked by something solid, no amount of force will make it engage properly, and raising the clutch will just grind the gears.

Workaround seems to be selecting any other gear, which it does without trouble, and selecting first again.

Any idea why its misbehaving? Its not common, maybe 1/10 times I start the car, and always on first start. Might change the gearbox oil soon to see if it helps.
Very first thing to check is the gear cables at the shifter end in the engine bay.
There is a how to within the forum.
Still seems to happen on the odd occasion, despite adjustment and plenty of grease. It does also occur with the engine off, if that changes anything.
That suggests the fault is not with the clutch but mechanical movement of the shifter in or out of the box