test

select 
    u.u_logon_name as UserName,
    ty.TaxYear as TaxYear,
    sum(el.item_qty) as Miles
from dbo.INS_EXPENSE_LINES el 
    join dbo.INS_EXPENSE_HEADER eh on el.expense_ref = eh.expense_ref 
    join dbo.INS_USERS u on eh.user_id = u.u_logon_name 
    join (
        select 
            line_id,
            case 
                when item_date < cast(cast(year(item_date) as varchar)+'-05-06' as datetime) 
                then year(item_date)-1 
                else year(item_date) end as TaxYear
            from dbo.INS_EXPENSE_LINES el
        join dbo.INS_EXPENSE_TYPES et on el.expense_type = et.g_id
      where
        et.s_IsMileageCode = 'Y' and 
        et.s_IsPrivateMileage = 'N') ty on el.line_id = ty.line_id  
where
  eh.status_id = '34' 
group by 
    u.u_logon_name, 
    ty.TaxYear