I dunno if any of the laws have changed but I used to work for The Link and regarding the 14 day change of mind that''s a service not a law.
It's basically up to the company to decide if they want to offer that service to clients and as far as I can remember T-Mobile stopped that back in 2005 but still offered exchange of phone down to the shops discretion.
14 day change of mind only applies to the distance selling act from what I remember so if you got this in the shop then unless the phone is faulty or they changed the policy you're basically screwed.
Regarding the battery issue sometimes the level of signal can determine how long the battery lasts for ie low signal strength means more of the power is used.
What you could try is if you can obtain a second battery and find that the battery lasts a lot longer than the current one then you could take it back to them and ask them to either change the handset or cancel the contract under the 28 day guarantee.
Hope that helps and if you need to know anything else just give me a shout