[twin plugged targa]

Hi guys, am stranded in the UK whilst picking up my new car-
87 930
symptoms: car either turns over and starts perfectly or
does not turn over at all
When it does not start, it drains the battery down to 8volts when trying to turn key

Have removed and load tested battery - battery fine
Have removed and abraded/Cleaned + and - terminals and attachments including some + connections to the battery which were a bit corroded
Have removed battery ground strap and cleaned bodywork connections and terminals
Car has just had a new starter motor installed by garage.
Alternator charges battery to 13.2 volts indicated on tester and battery holds charge

I am going to try and start the car tomorrow after trickle charging battery and hope to get there without stalling/ needing to restart- fingers crossed!

Any tips of what to check apart from new starter motor/ starter motor attachments?

Many thanks
Ben