My battery has been going flat intermittently for a few months. I got a new battery a couple of months ago but recently it's been happening more and more, to the point where, if left overnight, it will need a jump start in the morning.

I got a mechanic out and he told me the issue was the alternator, so I got a new one installed. The next day I had exactly the same issue. I've since done tests to check for parasitic draw but there isn't any. The mechanic advised that it must be the battery (having been worn out by the alternator issues and regular jump starts). I took it back to Halfords this morning and they tested it and told me the battery was in perfect health. Eventually they did agree to replace it, but I'm now concerned there is still an issue but I'm at a loss to what it would be! Any ideas if could still have been the battery or what else it could be?

Context:

Honda Civic 2005

The car will start if it's been driven within a few hours, but not typically if left overnight.

If it won't start it seems completely dead, where the central locking doesn't even work

When needing a jump start, it will need to be left charging for a few minutes before I can turn the engine on.

When it starts it does so straight away, with no odd sounds.

No other electrical issues detected.

Any help would be much appreciated!