Had another look at the linkage today, but unfortunatel I'm no further forward.
This is the distance betweeen the bottom of the c-shaped bracket at the end of the gear change mechanism - shown here in 1st gear...

This is the measurement when placed in 3rd gear...

I tried attaching the column shift to the mechanism while the gearbox is in 4th gear, but as soon as I move anything it can only find 1st and 2nd.
There are a couple of things I wanted to check, firstly the engine is about 4-5cm off centre - looking from the front of the car, the engine would be slightly towards the left. However the distance from the gearbox to the suspension towers seems to be equal on both sides - is this all normal?
The gearbox stay is leaning backwards quite a bit too...

It would appear that the end of the column shift isn't out far enough to engage 3rd and 4th - and it also looks like the shaft is off to the left (looking from the front), like this...

which obviously doesn't help - should it be in the centre?
I took some things off to investigate why the shaft wasn't in the centre - such as the bulkhead bush, which was quite worn...

I'm assuming the end of the steering shaft looks correct?

or should it be further down?
These were the bolts holding the sliding part of the mechanism - one of which is obviously not factory and seems to have done a great job of stipping the thread in the housing!

And this was the top of the housing (please excuse the dust!)

No sign of the pins that are meant to be in there and there seems to be a screw missing from the top of the column-change shroud, which might not help when it came to keeping it central to the bulkhead?
I've got a spare steering column, so I'm going to try and fit that instead.