I tried to find an unlock service for the 4030x but failed.
I paid an ebay service but they returned my money say they couldn't do it.
In the end I rang T-Mobile and paid £20.49, they said it would take 10 days.
After 10 days they sent me an SMS saying it would be 10 days (20 days)
I rang them and complained, but it still took nearly 20 days.
They sent me an unlock code which worked perfectly.
I then inserted my GiffGaff SIM and all was OK.
No more rip off PAYGO data charges.