If that was happening where I live, I would suspect poor drainage--or being allowed to sit in it's own drainage in a cache pot--resulting in a sodium buildup, and/or low humidity. In Wales, I dunno. I imagine that there aren't too many places in the UK that have low humidity, and high sodium in the tap water. Excessive fertilizer can mimic high sodium levels, and will also build up with compromised drainage.

Thank you, it could be too damp/poor drainage. Being a fern I thought it liked humidity so I've been giving it lots of water. I'll repot it.

Most ferns like high humidity, but most also need good drainage. I like to pot mine up in African Violet Mix, since that has good water retention, but also a more open structure than regular potting mix.

Thank you, I'll get some of that compost.