Top end rattle on 200tdi Disco - worsening

Hi Folks, have just joined this forum and am hoping someone can help with advice on a problem with my 200tdi.
About a month ago I started to hear what I thought was a noisy tappet. Checked and adjusted valve clearance - none were vastly out but the tapping has turned to a full blown rattle.
To give a further clue the engine starts on the button and for the first few seconds there is no rattle - it then gradually creeps in and whilst driving away cold will make you grit your teeth. It had improved when warmed up but that is no longer the case. Are these engines prone to breaking tappets or wearing cam followers which is what I suspect is happening?
If I haven't bored you to death by now I welcome any suggestions.

Well - pulled the head off today and have found the following....
1) pistons and bores look great - still has the cross hatched glazebuster marks in the bores - not bad for 116k.

2) All the tappet slides, rollers and housings are fine.

3) Tiny little valve caps are fine

4) HOWEVER - i think I have found the problem - at the top of one inlet valve there are three grooves and two lands where the collets sit. On this one valve the the lands have nearly worn away so the valve was bouncing a fair few thou within the collets. What drew my attention to it was when the valve did not want to come out through the guide and had to be punched through (top had mushroomed slightly)

Glad I pulled the head off otherwise I'd be picking the pieces of a dropped valve out.

I seem to recall stories of 200s having troubles with cam followers on cylinder 3 or 4 but its been some years since I had one .