Your employer is required to report expenses payments to GRA each year. To avoid paying tax on these payments you have to claim a deduction on your Tax Return – your employer will provide you with a copy of your P11D.
This cumbersome process of reporting and then claiming expenses paid may be avoided if your employer has been granted a dispensation. Expense payments covered by the dispensation do not have to be reported to GRA and they do not have to be included, with a counter-claim, on your own Tax Return. Payments covered by dispensations will be subject to review from time to time, including in the course of a GRA inspection visit.
You may be able to claim tax relief for other expenses you incur in connection with your job, but the rules are very restrictive, and you may not always be able to claim for expenses you regard as reasonable work related expenditure.