CGT & shares cost re-allocation from M&A event

[kiwilondon99]

Starting to put tax + CGT data together
Tax Accountant from top tier firm - so to answer a tiddler question will cost me a bomb [ know that from past !! ] Net tax impact effect to me is zero tax impact/cGT on this overall
As noted below - inteerested in how costs are reallocated from a CGT event

BACKGROUND : So - Invested personally - in an AIM co [X} 10 years ago. 35k shares £ 1800
2 M&A transactions back to back in 17-18
A] demerger assets to NEW CO – an asset sold from X into A, a Canada TSX listed giving 850 new share[0.0243 new per old]
Plus back to back
B] rump of company assets left, became a sub of an unlisted Canada business on 1:10 in new co via some Canada scheme of arrangement etc. NOW: Note no holding no value on the Original AIM investment [X}

Trying to apportion the costs to new holdings :
So 850 Listed [A] & reallocated cost now for these is = ?
And 3500 unlisted [B} Canad also & reallocated cost is = ?
Reallocated from Org [X] total £ 1800
And what is the CGT position on A

So how does one allocate base cost to these 2 new holdings – whats the mechanisms?
Interested as the Holding in new LISTED [A] above is to be taken over in 18-19 – so need some senible cost
Again-sadly due to such minor size- unlikely to cause any CGT 18-19 return impact but is CGT event – but Cost element calculation/methodology is of interest to me
Just really interested in how the mechanisms work for allocating individual holding costs ?

many thanks