Thanks for your new question and asking for me.
As these expenses are all wholly and exclusively in the performance of your duties they are not liable to tax - so are you paid x expenses then its topped up to £3000 OR are you paid £x expenses in addition to the £3000 dividend
If you are paid £3000 regardless of any reimbursement of expenses than my initial answer stands, if you are paid expenses then topped up to £3000 then this will alter my answer a little
Do advsie further
Then these are not business expenses these are travel to and from your place of employment enabling you then to actually perform your duties.
These are NOT allowable employment expenses
So you are liable for consideration of tax on the £8000 not the £5000
Thanks for your further question
You ask for more details? What more details do you need?
Until you arrive in Qatar are you able to start performing your duties as you advise this is where you undertake all your business. so any expenses incurred getting to that place are not business expenses from which you can claim to reduce your tax is this is your regular place of work so this is deemed normal travel to and from work along with all the associated costs you incur in that 2 week stay.
So you are liable to tax consideration on the full £8000 with no deductions for this travel as it cannot be accepted as business as NOT incurred in the performance of your actual duties but in fact incurred so you can be in a position to actually perform your duties. (and this is the difference)
The expenses are not business expenses