Have had problems recently.  Having looked in all the catalogues I am now totally confused, we seem spoiled for choice (D12, V12, Holt ultraline etc).  !What is the best non stretch rope to use - 2-1 or single part?

Marlow D12 or Holt proline are sufficiant. Marlow V12 / Ultraline is an unecessary expense.

My current main halyard looks strong enough to moor a battleship - what diameter/gauge do I actually need?

I have 4mm for my halyard

Most people are using 4mm Excel Racing or equivalent.  Does not need the ultimate in low stretch and the loading is not excessive either - how hard do you pull on the cunningham?  Not more than 40kg max and likely no more than an inefficient 4:1 purchase so 160kg max static load and double it for breaking strain (or 1600 newtons for the engineers if memory serves).  Excel racing will do the job.

The D12 i use is 3mm. I use a similar setup to the old Jon Turner CTs with a wire halyard in that i have the load taking bit as D12 and then a spliced on tail which is whatever is cheapest. The D12 is tensioned with a 2:1 purchase at deck level to (usually!) stop it slipping which is a real problem with conventional Spectra/Kevlar halyards.

(long time no post!Rather than the cunningham, I find that the kicker tension is the thing that pulls the head of the main down. I have no idea what the tension a 16:1 kicker full on is and what actually gets converted to downward force, but it is often enough to make halyards slip through slightly worn cleats.

Exactly - it's the kicker that causes the problem.  And its not a slipping cleat as I use an old fashioned horn cleat.  I used to use an excel racing 3mm 2-1 and that worked fine.  But a 2-1 is a hassle and lots of string so I wanted to revert to a single part - hence the question?  Looks like 3mm D12 is worth a try.

You don't need the full on 2:1. The system i described uses 2:1 purchase on a separate tensioner. It's not very clear, have a look at this photo.

I've tried horn cleats, double clam cleats, clam cleat with horn cleat behind and they all slip.