it's a wonder when your phone was reported lost and a stop put on the SIM that customer services didn't send a replacement to your UK registered address at the time.
You will need to contact them again (+447953 967 967 from abroad) and request a replacement.
Your mum might be able to make the call for you (0345 6000 789 from the UK) given your mobile number and password but normally I would have thought such requests must be made by the mobile account holder personally.
Let us know how you get on
If you want to say thanks > click 'Kudos'. Answered your question? > click 'Mark as Helpful Answer' also.