Gentlemen, I don't enjoy sharing my misery with you all on this one but i'm glad it's got you good folk equally perplexed
Much to digest from your comments - many thanks for those - so i'll try and narrow things down a bit.
12-25 Cassette (Campagnolo CS-8AL), almost new - discounted as an issue. Chain, 8-speed, brand new, installed so that when on the big ring front and small ring rear the jockey wheels are vertical - discounted as an issue. My lack of patience - not new and could be an issue
So that leaves RM, DT levers and cable as the culprits. Are they all in on this together or is it just one of them?
Cable is brand new - Jagwire Road Pro - use this cable set on all my bikes, never had an issue so may not be an issue but not discounting it. A more traditional cable may be better? It is held via the traditional groove on the RM - does this influence it's operation?
RM I bought from Holland a few years back, in lovely condition, it's clean, no dirt, I'd be amazed if it's done a 1000 miles looking at the jockey wheels. However, I must agree with the comments that this may be 7 speed maximum as I can't see anything official that says it can index to 8 cogs despite the ability the 'travel' across the width of the cassette. Prime suspect #1.
DT levers - 7 clicks and a 'ghost'. I can't see anywhere that shows this lever set as 7-speed or 8-speed variants - the ferule that holds the cable end only appears on the Record levers - I had taken this to mean these are 8-speed only but there seems to be some conjecture on this? (Campagnolo SL-01RE CG, Record). Prime Suspect #2.
So - to the symptoms explained a bit better:
53T front ring - 8th (smallest rear) - sweet like chocolate as is 7th, 5th, 4th, 3rd.
53T front ring - 6th - it 'clatters' - no interference with the jockey cage so this isn't a metal on metal problem.
53T front ring - 2nd - doesn't like it all (see comments below for small front ring)
53T front ring - 1st - can't comment - it's never been there but as I wouldn't ride with a chain cross like that I'm not concerned other than I've never had a RM that won't go across the rear gears before.
39T front ring - 8th - can't comment - it's never been there.
39T front ring - 7th - It's not very happy and clatters.
39T front ring - 6th, 5th, 4th, 2nd, 1st - sweet like chocolate.
39T front ring - 3rd - prefers it to 7th but again it's not too happy.
What I've summised is that the RM isn't 8-speed compatible in index mode - the DT levers are index only - I have others but none that are 8 speed
I do have the get-out clause of a pair of friction only (C-Record I think) but a primary driver on this build was for it to be early 90's set-up so index shifting was high on the wish list.
Also, which is setting A and B - i've got the allen fixings on the RM just tight enough to allow the RM body to flex across the width of the cassette and I'm using 1 limiting screw only - It's limiting to stop it going beyond the 12T cassette cog - the other end is controlled by the allen bolts so it doesn't seem to need the use of the other limiting screw? Not sure if this is correct but It doesn't seem quite right to me to be doing it this way?
Any finally to the DT internals - see attached - to my eyes it doesn't match any of the colour coded inserts so 8-speed??
So then gents, far too many of my words and I haven't fixed it so any thoughts on the above?
Rick Powell 753 Track / Ribble Gran Fondo / Colnago Asso / 1975 Harry Quinn TT / 1990 Concorde Prelude / Vitus 992 / Keith Coppell Lo-Pro / 1986 KAS Vitus 979 / 1995 Mercian Strada Speciale / 2011 Raleigh GAC Militis 3 ex Graham Briggs / 1995 Ribble 7005 / 1986 Alan Super Record / 1985 MK1 Vitus 979 /
On the bench: Sirius 853 TT / Sun Solo Chrome / Ciocc Designer 84 / Giant TCR0 Pro Compact / Ribble 653 Tourer / Daccordi Lo-Pro / Look KG96 / Bottecchia Equipe / Look KG231 Credit Agricole / Contini Grand Prix / Aende Lo-Pro / TVT / Langdale 653 Pursuit /