Generally, the preferable basis of computation of profits is one in which
Items (a), (b) and (c) must all be adopted for the basis of
computation to be acceptable.
Unless the trader wishes to change to the basis described in
the first sub-paragraph above, there is no need to disturb other
long-standing methods of computation (for example, where the
service charge is spread over the period of payment and no
deduction is made for agents' commission on outstanding
instalments).
Where the basis of computation is changed, you should make
adjustments to ensure that the new basis is applied consistently
for the year of change, with opening balances adjusted on the same
basis as closing balances. For example, where a claim is made for
the first time for a deduction in respect of agents' commission on
instalments outstanding at the end of the trading year, account
should be taken, on a similar basis, of the amount of such
commission at the end of the previous year.
For further guidance on this point see
BIM34000 onwards