Keep the 1 month contract
15/month, 350 minutes, 300 texts, unlimited internet
£350 phone second hand (can find them "as new" for this)
Total cost of ownership 2 years: £710
LEAVE AT ANY TIME

Get a 24 month contract
30/month, 900 minutes, 3000 texts, NO INTERNET
£0 phone new
Total cost of ownership 2 years: £720
Locked in for 2 years

If you really talk that much, and wanna tie in for 2 years then go for it. Adding unlimited internet to your 24 month contract will cost another 120 for the 2 years