Right, I take back what I said about the samsung service centre. They're useless and wasted my time. As far as I'm concerned this is no different to before the 'repair'. Same audible rattle on left side when put close to the ear, and same unresponsiveness unless you press very hard. They said they'd replaced the button. Well that might be so but they fixed nothing imo. Then they said that the pressure sensor is more to the right implying the left side isn't as sensitive and then he said you're meant to press it in the centre, effectively implying I'm an idiot. I can press the very right edge of the button with the lightest of presses, just till it clicks, and it will respond. It won't on the left side. Can someone here test theirs. Press the very left side of the home button with a very light press just till it clicks and see if it responds as well as as the right side.
There are other service centres not too far. I'll take it to another. Don't get me wrong, the button is fine generally, but it's not as it should be. The chap that sold me the phone at the EE shop (who I went to first today and who then put me onto the service centre) said the button wouldn't bother him. Am I being too picky?
edit: oh, and they gave me back the phone fully factory reset. Thank god I'd backed everything up because they hadn't warned me in advance they'd reset it. Amateurs.