Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question.
It is possible that your tax code is incorrect. Your employer must apply the code as notified to them by HMRC. You should receive a copy of this code also, but it is not unknown for the employer to be notified and the employee not and vice versa. Furthermore HMRC admit that thousands of the codes they send out are wrong.
On a salary of GBP 815 per month you are below the personal allowance level of 10600 per annum so, providing you have no other source of income, you should not have any tax being deducted at all although if the monthly income fluctuates you may be taxed one month and get it back again when the monthly income falls. However the 815 is not far from the normal monthly start level for tax, 883, so you could be a borderline case if your tax code is below 1060..
What you should do is to check the tax code sent to you by your tax office. If it is wrong write to the tax office which issued it and ask for it to be corrected. Tax codes do a very effective job of making individuals tax neutral at the end of each year, but only if they are right in the first place.
Actually, I do not think your employer is far out in their advice, just compare the two figures I have given you.
I do hope that I have helped you with your problem.