Contact ex employer over pay

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Hi,

I worked at river island for a bit over christmas as an X-mas temp. I've now left, but have only received about half of what I should have been paid - it's now nearly a month since my last day. I was paid about half of what i was owed. After speaking to permanent staff, it seems that contacting the shop itself is useless for stuff like this - who do i contact about this? The account I have for the website they use to pay us closes in the first week of Feb so i would like to sort this quickly.

Thanks