There is some detailed information here and here on the tax implications of the Vodafone/Verizon demerger. As you election cash dividend, you took the C option.
The adjusted cost of the Vodafone holding of 44,622 shares was £19,634 (44p per share). The new Vodafone holding was worth £58,998 and the new Verizon holding was worth £33,161 after the demerger. The combined value of the new holdings was, therefore, £92,159. The cash received from Vodafone was £13,187 but it is a dividend so has no part in the cost apportionment. The cost of £52,795 (£19,634 + £33,161) is apportioned as follows:
24,339 VODAFONE SHARES
(£58,998 / £92,159) x £52,795 = £33,798
1,173 VERIZON SHARES
(£33,161 / £92,159) x £52,795 = £18,997. Any cash paid to your of Verizon shares should be deducted from the figure of £18,997 to arrive at the true net cost of the Verizon shares. Assuming no fractional cash was received, you made a gain of £13,958 when you sold the Verizon shares (£32,955 - £18,997).
The £13,187 return of value payment and the value of the Verizon shares of £33,161 are treated as dividends tax purposes with a 10% notional tax credit attached so the gross is £51,498 (£13,187 + £33,161 / 0.9). You will almost certainly have higher rate tax to pay on some or all of the dividend depending on how much other income you received in 2013/14.
I hope this helps but let me know if you have any further questions.