Small amount of brush in grey patio groutio (Mickleover DE3)

I had a patio put down about 4 years ago but about 3 sq meters of grey grouting had crumbled. I have about half a tub of grout left over from the job and decided to have a go at digging the old stuff out and redoing it. I've been at it today but have run out. I only need about 4 large trowels worth, it's over £40 a tub and It will just be wasted. Really grateful if anyone can help