What's happened is that the ****ty stereo that Alfa fitted from new didn't draw power when turned off, so the live, red wire behind the head unit in the cars loom is live all the time, no matter what the ignition is doing.
A modern head unit will draw power even when switched off by it's power button, so it needs a switched live, ie one that's live only when the ignition is switched on (to power the unit) and a permanent live, one that's on no matter what you do with the key (to store radio channels), this one draws very little power compared to the other live wire.
What you'll find is that someone has connected the cars red, live all the time wire in the loom behind the head unit, to the wire on the head unit that only needs power when the head unit is switched on, this draws 0.40 amps and flattens the battery over a few days.
I found that the wire that needs power all the time to store the stations was ok.
So what you need to do is..............
Flip out the gear lever shroud, use a blunt screwdriver on the front edge to prize it forward and up, then one side, forward and up so it pops the lugs out of their holes, under it, to the left is a multi connector, connecting up about 10 wires.
This connector runs under the centre console and powers the ciggie ligther and mirror adjustment switches.
One of the wires that run through the connector is orange (it's thicker than most of the others), this is the switched live, ie powers on and off with the ignition key.
Once you find this, disconnect the red wire from the head unit to the loom, tape up the loom (car) end of this wire, you don't need it.
Connect the power lead from the head unit (the wire you disconnected but didn't tape up), it's usually red (if you have the head unit instructions, it will be called "switched live"), to the orange wire you found running through the multi connector under the gear lever shroud.
If you plug the head unit back in, it should now only work with the ignition turn on.
If you haven't disconnected any other wires to the head unit, it should save the stations when you switch off the ignition.
If it doesn't, there should be another wire from the head unit (usually yellow) this needs power all the time, connect this to the cars red wire you taped up earlier, but you'll ptobably not need to do this if the rest of the head units wires are ok.
Note, try not to pull at the multi connector to much, as it's fragile.