Capital allowances for sole traders are available for a 12 month period. These are adjusted and pro-rata for a short period of accounts – say 6 months accounts would mean 18% x 6/12.
In this case the accounts are 15 months long, so the adjustment for the CA is 15/12 which are then deducted from the net proift – then you would apply the basis period rules to see which tax year taxes which proportion of these accounts.