Is it illegal for a company to not pay part-time employees for bank holidays?

I work for a very well known retail company on a 30 hour contract. In the new year they changed their payroll system and there has been issues ever since.

I was not rostered to work for the bank holiday in March and as far as I’m aware I’m meant to receive a fifth of my weekly wages for this day. It’s been over a month since this and I’ve informed my manager who told me to contact payroll. Thing is, they only have an email. No phone number or anything. It’s been a month, I’ve sent 2 emails and haven’t gotten a response.

I’m not quite sure what to do now and management hasn’t been very helpful.