The breakdown came out and jump started the car, tested the battery, and said it was fine.
He said not using the car for long journeys was causing the problem, but we are in a lockdown, essential travel only.
I have the car booked into a garage recommended by a friend, to have the knocking looked at.
I am starting some voluntary work delivering food parcels and NHS supplies next week, so fingers crossed it's not a big problem.
I live in Manchester.