Foreign share sale, stock spin off - query

Could someone please advise as how to deal with this example situation: I hold shares in a company that has undergone a spin-off, and I now sell the original company in a single tax year (or the spun-off one, for that matter, but not both). So, for example, I purchased 100 shares in company A, for 50 Euro each (5000 Euro total), several years ago. Company A now issues shares of a new company B, providing 1 share of the new company per 2 shares of the old, so that after the spin-off I still hold 100 shares in company A, and 50 shares of company B. If now, in a given tax year, I sell all 100 shares in company A for 6000 Euro total (60 each), still holding company B, what cost basis should I use? Would it be correct to just use the 5000 Euro original cost basis, for a profit of 1000 Euro on which to pay Capital gains this year, and then when in the future I sell the shares of company B use 0 Euro as the cost basis? Or should I somehow split the cost basis between the two?

Hi
Please refer to guidance at:
Shares and Capital Gains Tax (Self Assessment helpsheet HS284)
Thankyou.