I'm surprised Gordon has let you get away with this for so long. Standard tax rules are that any 'allowance' is a payment from employer to employee, and is therefore income, and is therefore taxable.
Re-imbursement of expenses is simply the employer paying the employee for expenses encountered 'wholly and necessarily' incurred in doing their job - hotel bills, meals etc. These are not, therefore, taxable.
To ensure that the expenses are genuine, the taxman may ask to see receipts. But in most cases (certainly the company I work for), the expenses will only be reimbursed by the company on production of receipts.
SSD