Altenator packed up

The started to pack up different lights have come on and the wife carried on driving til it packed up completely, as though the car was trying to get home but the electrical power needed at night was just too much for it.
The last thing she saw as it stopped was altenator failed

The car was dragged onto the tow truck, I told her to get the guys to call me but they knew best.

When the car was dropped off I disengaged the parking and gears so the car could be rolled into my garage.

I thought check battery, FLAT, but it was replaced a year or so back as kept getting "recharge battery" common fault but with the amount of juice these cars use for an hour after leaving them !

Anyway suspect Alternator to be duff.

Wanted to start car to check voltage from alternator cheaper to replace battery first maybe

Battery measured 12.30v so put an charge as ampage may be low
Measured whilst on charger still 12.30v going to 12.63v, tired jump start, other car kicking out 14.00v but when connected to jump points drops to 12.30v
So disconnected alternator
now battery getting as charge of 13.70

car will NOT start.
Cleared all reported errors using carsoft diags

on trying to start

noticed that when I put foot on brake press start and can hear clicking from engine bay but nothing. Take foot off brake press start and get the same clicking sound but NO "Press brake to start engine" warning appears unless I rapidly do the above in succession.

Can the car still be in gear even thought it says its in park on the gearshift?

I would like to repair the car myself as I have had far too many issues with my stealership. I am thinking its an alternator diode that has gone, could smell burning like diodes. Why give them the satisfaction of a large wedge of cash when I can buy myself a nice pint after completing a nice quick job.

so over to maybe someone who can give me a bit of guidance