Are they dying? Or are they just the spore producing fronds which some ferns grow?

Are they the oldest leaves or relatively new ones?

The leaves that are turning brown are in the centre and are as far as I can tell the new ones? Is it suffering from not enough watering maybe?

It sounds like a moisture problem. As we've had a prolonged dry spell that might be the issue. Browning of fern leaves is either caused by excess dryness at roots or the fern leaves getting too much direct sunlight.

Bendipa- thank you for your suggestion. Will try and water it regularly. Could I cut out the dying leaves?