2cm gap in staircase stringer

[SJRSJR]

Hi. Hope you can help with some ideas. There's a 2cm gap between the wall and the stringer at the foot of the stairs which gradually to 0 at the top. I'm wondering how I can make this look acceptable. I don't think I can use beading or trim as the stringer curves at the top (unless there's a very thing flexible one?). I've raked out the silicone sealant that was there previously and want to find a decent long term solution so any ideas would be appreciated. Thanks

 

[Notch7]

a bit of a long shot but this is an idea:

https://www.decorative-coving.co.uk/shop/coving/flexible-coving/cx132

 

[SJRSJR]

Thanks for that. I found some 4mm wood trim which had enough flex in it. Rather than cut the wood down I cut into the plaster and used panel pins to attach it to the stringer. I then taped some masking tape to it and filled the gap first with expanding flexible foam then finished off with filler. When it was dry and sanded I removed the masking tape leaving a hairline between the filler and the trim which I hope will mean the filler won't crack. I've not finished it yet so I'll see how it goes before I paint it. Work in progress photos attached.