Hi,
Yes, also the MKIV head on my engine is made by “Chromidium”, but almost all other alu and iron castings in my engine have the “Repton” logo, except timing case cover and a few other bits that have the “WM” logo. I thought it was for “West Midlands” foundry, but I guess I was wrong as on the XK forum Roger Payne could tell it was for the “William Mills” foundry.
And now that I have my engine running again and the car back home, I’d like to thank you all who have helped with your comments and observations and sharing your knowledge and experience. 
And I’ll be glad to post more photos of anything you’d like to see.
I am now pretty much convinced that Roger McWilliams had/has the right conclusion about what went wrong: oil starvation in the upstairs department.
Now that I have been looking at the old rocker shaft and pics of the rockers and valves before the rebuild it is obvious that:
A) In my cylinder head, the valve guides were not original and they were not original spec either. They were cast iron, possibly from some American car that shared the dimensions for the valve stems.
One got broken in the engine breakdown. I think the pieces that I found after the first start-up after the rebuild inside cyl nr. 6 and in the inlet tract to valve nr. 11 are from this broken valve guide in cyl nr. 4 (valve nr. 7, exhaust valve).
B) When I took the cylinder head apart I noticed that many (not all, but maybe 60-70%) of the valves were slightly stuck in the guides. I had to use more force than normal to force the valves out of the guides. At the time I thought it was because cooling fluid had entered the head though cylinder nr. 4.
but it may be this was due to oil starvation in the first place.
C) Now that I have (thanks to your suggestions and Alan Gibbins in the UK) new wicks inside my rockers and a brand new rocker shaft and new bronze bushes reamed to spec on it and I could see how nicely the oil holes in the shaft aligned with the rockers.
Not the case in my old shaft! I have no idea how it’s possible that many of the oil holes have been covered by the bushes, in some case the shaft has a “flat” so maybe someone tried to make it work that way, so the oil would find it’s way onto the rockers?
D) The old felt wick inside some of the rockers was a very hard black thing that was difficult to take out. Those were the same rockers that showed considerable wear in the end that touches the valve stems.
Also it puzzled me a bit that on some rockers I could easily push the new wick in with my fingers, but a few were so tight I had to use pliers and a Torx 20 screw driver to push them in (and in the second to last case at 11 PM at home I accidentally pierced my LH middle finger with that tool! Outch! But thank God, it’s alright, just some blood spilled, but no permanent damage) so it would appear the rockers are from different batches.
Also the cast numbers on them, (below that tilted rectangle) range from 4 to 24.
And to top it off, I appear to have SEVEN pcs of C437 and only FIVE pcs of C438.
I put it back that way, but as also pushrod nr. 10 is very slightly bent (I could see it when I spin in, that it is not perfectly straight).
Anyone know where I could easily source a new or good straight second hand pushrod and one C438 rocker?
So in the near future I would replace those two items (one rocker and one pushrod) in my now nicely running engine, perhaps in the spring when I would retighten the head and then I would need to remove the rocker shaft anyways. 
Cheers!
Ps. Good timing to get the car home on Saturday, as the weather changed a bit yesterday and now looks more like this:
(darn iPhone, I don’t know why it ends up coming up sideways…)
