Is your contract stated as an annual wage or hourly rate? If its annual the payroll department would just divide that annual wage by 12 or 13 depending on how many times in the year you are paid (monthly or 4 weekly) therefore you would have same monthly wage every year..,, eg £18,000 annual divide by 12 equates to £1500 per month.
If its hourly rate they would calculate the hourly rate by the amount of hours you work in week then times by 52.177 then divide that figure by twelve to get monthly
Eg. £8.50 x 39 =£331.50 x 52.177= £17296.675 (annual salary) / 12 = £1441.389 monthly salary
Some companies might only times the weekly rate by 52 which means the annual salary would be £17238 which would give a difference of £58.675 per year and £4.889 per month
You need to look in your contract and see what it's stated and what hours your contracted etc to get more definate answer