Hi Martin, Ouch that looks deep
This is what I use for jobs like this,
https://www.amazon.co.uk/Novol-FINIS.../dp/B007Y32U3G
It dries to a light colour and is easy to wet sand to an ultra smooth finish. You might be able to mix some paint and touch in with a brush but I suspect you would get a better result by spraying the whole panel.
If you like I can lend you a tin of filler as I live just around the corner from you.
PM me if you do.
Kind regards Tony