Good to see an interest in the finer details of the accounts.
The accounting of the value of player registrations is slightly less opaque than you are considering.
Each registration is booked in taking to account the total cost of acquisition including agent fees etc. This appears as an intangible asset on the balance sheet.
Each asset is then amortised over the life of the contract.
So in the 2014 accounts they start with the previous year's net book value, they add in acquisitions (31 million as you mentioned) and reduce by the amortised figure of each player disposal.
They then further reduce this figure by the total amortisation of all players contracts over the year to arrive at a net book value for the total playing staff (the intangible assets)
There is a further revision of asset value if a players contract is extended to reflect the newly revised contract life (length)
The figures are in item 10 of the accounts
Hope I am not teaching grandmother to suck eggs