Hello and welcome to JA!!!
I'm Billy and i will assist you with this.
First i need you to confirm that you have a manual gearbox.
The grinding happens when engaging the gears, or all the time?
do you have any other symptoms?.
was anything checked so far, or any work done lately on the vehicle?
Get back to me on these and we will find the answer for your problem....
The noise continues all the time when running in those gears.
The noise continues when running in all three lower gears. No work has been done on the car recently
It is a manual gearbox
The first thing you should check would be the gearbox oil
That might need topping up or even replacing........it would be cheap to drain and refill.......(if not up to norm).
If that resolves your issue......then you can keep the car.....as it would function normally.
However, if that does not fix the issue......then most probably you need a new gearbox......and then you may consider getting rid of the car........although the gearbox can be replaced.
the manual gearbox you have takes two liters of: 75w-80 semi synthetic oil, which should not cost more than 20 pounds......and the replacing would show if any metal pieces drop with the old oil.......suggesting breaks in the gearbox....and thus the need to replace or even getting rid of it....