Backdated JSA Payment

I applied for JSA in November, but was made unemployed in October. I had my meeting with my local officer in December and gave her all the paperwork, etc. I also informed her that I had just found full time work for the Christmas, so I was only looking for a backdated payment from October to November. She said this was fine. And she would suspend my application, so I could easily reapply for JSA in January when my full time work ends. So I got a letter in the post on December 14th, saying a cheque will be issued to me on December 15th with my backdated payment. But it didn't say how to get the cheque. I presumed it would come in the post, but there's still no sign of it and today's the 22nd December. Am I supposed to collect the cheque in the post office? I tried calling my local office but nobody ever answers. And I don't have time to call into my local office or post office to cheque because I'm working 09:00 - 18:00 Monday to Saturday. Anyone know how the cheque is issued?

Thanks 🙂