My employer required 4 weeks notice. I sent them my resignation email on 01/Dec/2016. The email said that my last working day would be 30/Dec/2016.
In Feb 2017 I received an 'Acceptance of resignation' email from my employer. It stated that they were accepting my last day of employment as 02/Jan/2017.
I emailed HR highlighting the incorrect dates compared to my resignation email. They replied saying:
I believe the end date for January was owing to the bank holiday on the 2nd January so even though you finished working in December, your contract end date fell on the bank holiday.
Was my employer allowed to change my last working day to a later date?
The reason this is an issue is because they are refusing to pay me for unused accrued holidays during the 2016 working year (Jan-Dec). My contract stated that I'd forfeit these holidays if I did not use them, or leave the company, within the same working year.
Update on 03/Mar/2017: I contacted ACAS and they advised me to follow the companies grievance procedure before taking any further action. After informal talks with multiple staff over the course of two weeks, they eventually agreed to pay me the money that they owed. As highlighted by user52889 below, it is worth acting quickly if you find yourself in a similar situation to mine as there is a three month time limit for making a claim and not completing all of the recommended steps beforehand could reduce the amount of compensation awarded to you if the case does end up going to an employment tribunal.
Thanks everyone for your answers - they were much appreciated. :)