With fully charged battery engine spins nice and quick but after a week or two starts too slow down and headlights start to go into candle mode all earth,battery straps checked and cleaned alternator output 14.5v at idle but is making a zzzzzzzzzz sound thinking alternator giving volts but not the amps?.

So u have a functioning alternator giving out a decent voltage but a bit high for normal idle speed, and if your volt meter is reading correctly.
Output current is dependent on the load drawn from the alternator, so a higher load equals a higher current and a quicker the charge, but not a full 100% charge, only a stand-alone charger will do that.
U don’t say how old your battery is, but a smart charger should revitalise a failing battery, also some alternators can be be repaired by simply replacing the diode pack.

I’m lucky with my disco if the engine starts after being stationary for three weeks, thats with a three year old fully charged battery fitted at the start, a fully charged seven year old standby battery lasts just two weeks when fitted to the disco, that why its now kept as a spare… just in case.
A five year old newish car I have will zoom into life after standing untouched after two months plus, thats the longest I’ve left it standing unused (during lock down).

Sorry battery is getting on a bit but have fitted to other vehicles with no issues also put new battery on does the same thing

mr glee said:

Sorry battery is getting on a bit but have fitted to other vehicles with no issues also put new battery on does the same thing

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Sounds like your alternator needs an overhaul, if you're not comfortable with doing it yourself, an auto electrician will quote a price ...

It's possible that there is a parasitic current drain so check that: https://www.wikihow.com/Find-a-Parasitic-Battery-Drain if it's high disconnect the alternator's main lead and see then cos even if the charging voltage is OK it can be a shot diode in the rectifier and it will be a drain through it.