My unknown fern is Pteris Cretica ‘Albolineata’, or Silver Ribbon Fern. It has not overwintered well and needs repotting and feeding, I will move it into the shade, and put the pot in a saucer to try and keep it damp.

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A fulsome history & description of Pteris Cretica
Caring for Pteris Cretica
thank you for identifying this fern – bought one a while back from homebase for 50p as it was almost a dead gonner but has bounced back beautifully – have always kept in indoors – now I see it is hardier than I’d imagined p.s. those links are very informative – have a great weekend
Well, I have left it outside but it hasn’t flourished – maybe just my neglect. I think it needs more protection over the winter than I have given.