First impression - 1470 disables 'number withheld' if you have it normally enabled. 1263 is, from memory, the code you dial to route your calls away from BT onto (I think) NTL. 0854 (0845??) is then a lo-call rate, & the following numbers are to a particular phone, (or in this case the ISP's servers), in the same way as your own number.
The reason for the first 1470 is so that Tesco PAYG can verify your account - you will have had to give them your own number on registration, I'm sure. (You won't be able to dial up from another number).