IS THERE A PROFESSIONAL SERVICE AVAILABLE FOR CGT CALCULATIONS ON SHARES ?

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Does anyone know please whether there is an internet based reliable confidential service who could make CGT calculations for the sale of shares ?
I need CGT loss calculations on the sale of Nat West shares owned for many years. I have tried asking an accountant but it was too complicated for him and after a year he gave up !
I wondered therefore whether anyone provides a service so that I could give them all the details and they had a "program" which just worked out an exact answer acceptable to HMRC. I need an exact answer as I want to set the loss against the profit on the sale of other shares.
The position is complicated as the shareholding has been affected over the years by scrip dividends, bonus issues, rights issues, possible indexation relief for some early years (but maybe not for a loss ?). and capitalisation issues.
If there is no such internet based confidential service then maybe there should be !
What do accountants usually do ? Do they always work out such calculations personally ?
Many thanks for any suggestions

[someone]

I suspect the real problem is that accurate data for all of the events going back (what I assume) is 40 years isn't easy to come by. There will be specialist firms that will be able to handle this - I would assume, for example, that the data would all be available via Bloomberg - but that's a 20+K per year subscription so only a specialist firm (with specialist prices!) will have access. (I don't have any suggestions but I suspect any of the big 4 could do this but I'd expect the fee to be large, possibly bigger than the loss you're crystalising!)

I don't know what sort of scale of shares and losses you're expecting. If it's say 10K of loss then I'd just make a best guess at all of the events. So long as it's reasonable HMRC aren't going to want to make the effort to correct what might be an error of 1K. But if you've got 10M+ of holdings and 100K+ of losses that you're crystalizing then HMRC will (I assume) expect something better than best guess based on what information you can find on the internet.

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Thanks very much "Someone". At least if there isnt any such service that is helpful. I think that the losses are about £10,000 or so. it is just a question of knowing accurately the amount of gain which can be realised against the loss . I really dont want "insult to be added to injury" by ending up paying any cgt when i realise a heavy loss.!