Hello
The steering wheel on my Series IV A is not straight and I have been investigating how fix it. The central boss unscrews but cannot be removed and now cannot be screwed up tight again either. Anu ideas to remedy this would be appreciated. Secondly, what size are the machined screws which are used to fix the steering column cowl and the indicator / overdrive stalks as I've discovered mine don't fit properly. Any guidance would be welcomed. Thank you.

I cannot resist saying that the problem here is the nut behind the wheel!

It sounds as if you have 'lost' the end of the threaded rod which tightens the adjustment for steering wheel reach.
The screws (like almost all such small screws on the Alpine) are 10-32UNF. So, they are No 10 size (approx 3/16" dia), and 32 tpi, which is a Unified Fine thread.
But, the 2 screws which fasten the lower shroud to the upper, by each stalk, are the rarer 6-32UNC variety, with csk heads. And the one under the centre is a self-tapper which goes into a spire nut.

If we go back to the beginning, here's what you should have done:
1. Prise out the decorative centre from the bakelite adjuster ring. The thin chrome ring and the padded centre come out - BEWARE you don't lose the three tiny clips which hold it in the bakelite ring.
2. There will now be seen a bolt head in the centre which can be undone with a 1/2" AF spanner and removed. Take off the bakelite and there is a steel fitting which fits onto a small threaded rod down the centre of the column. But you can't yet remove this fitting.
3. You have to remove the horn ring assy by undoing the three 10-32 screws which attach the copper back-plate, but are accessed through the ring hub. As you pull away the ring assy, you will find a large spring which is used to earth the horn ring to the steering wheel hub nut. You also have to undo the wire from the lucar connector on the slip-ring. To get at this, the wheel must be 'out' (or back) and the lower column shroud removed.
4. Now, you can get at the centre fitting - in your case it will probably fall out. Behind it, there is a thin (5/16"?) threaded rod down the centre. You have to retrieve this - maybe a length of plastic pipe or tweezers. Or a piece of rod and super-glue. There should be a nylon circlip to stop it falling away, but this can get broken or lost.
5. Now you have access to the wheel's nut. Remove it and bang on the back of the spokes with your hand. It should come off.
6. Reassembly is the reverse, but having tightened the wheel, you have to fit the adjuster centre onto the threaded rod - at the far end it has a taper which locks the inner column to the outer.

If you can't get the wheel off, you can re-centre the wheel by adjusting both the two track rods by equal amounts. However, the two rods should be of approximately equal length. So if your wheel is, say 10 degrees to the left when travelling straight, you will need to shorten the left rod and lengthen the right equally - probably by around half a turn. If this will make the rods more than about 2mm different, you will have to try harder with the steering wheel. Try driving around with the nut half a turn loose (you can leave off the horn ring etc).

OK Chris I asked for that !

Many thanks for infomation and will give it a try.

Is there a known method for freeing off the reach adjustment on these? I can unscrew the boss (and tighten it again), but the wheel doesn't budge - I suspect it's because it hasn't been moved in decades. I'm quite afraid to give it a thump, in case I break the wheel!

The boss is attached to a nut which pulls on a rod which has a taper at the far end. To release it, you not only have to undo the nut, but push the rod to disengage the taper. But if it doesn't release with a light push on the boss, there is a risk of breaking the bakelite. So I suggest you take out the centre trim (not losing the 3 clips) and then hit the bolt head in the middle.
If you have unscrewed the boss a couple of turns, the boss should fall back about 1/8".
If the boss and rod are loose, then the inner and outer columns are probably corroded together.