No Capital Gains Tax is payable on disposal of shares after three years, or three years after commencement of trade, if later, provided the EIS initial income tax relief was given and not withdrawn on those shares.
So for example, if you had a gain of £10m, you could invest that in a basket of EIS and defer the entire gain. The gain will be treated as being realised in the year in which you exit the EIS.
You are able to reclaim CGT already paid on a gain that you incurred up to 3 years before you invested in the EIS and any gain that gets realised up to 1 year after investment you are able to defer through the EIS. This allows you to reorder gains and losses where you have incurred a gain and then in a subsequent tax year you incur a loss.