I had the same as you, but they confirmed this afternoon. Your experience is completely normal!Sent my phone to Argos by special delivery on Friday, saw they received it at 8:30am yesterday but no updates yet.
I used the address that Royal Mail gives for them when doing post code look up through their website. I also dont think I put the order/reference number on the outside packaging but wrote it on the box inside and put the received letter in there as well.
This is what the Royal Mail site gives:
Likewize Services UK Ltd
Unit 1-2
Crewe Logistics Park
Jack Mills Way
Shavington
CREWE
CW2 5XF