The general rule for employees' expenses: wholly and exclusively: apportionment
A single expense can be apportioned if a definite part or proportion of that expense can properly be attributed wholly and exclusively to the performance of the duties. That part can be deducted while the rest cannot.
For example, an employee’s car may be used for both business and private purposes. Where it is being used to make a business journey the cost of that journey is incurred wholly and exclusively for the purpose of performing the duties of the employment and so can be deducted, see EIM31845. For 2002/03 onwards see EIM31330 onwards.
You cannot apportion expenditure where it serves two or more purposes concurrently and one of those purposes is not to do with the performance of the duties. The decided cases that support this conclusion are summarised at EIM31662 and EIM31663. There is an exception to this rule where the non-business purpose is merely incidental to the business purpose, see EIM31664.