Have never had a banana tree so I'm only guessing that if it's been kept outdoors during the summer it has attracted some kind of mite, or leaf scorch from sun shining on it after it has been watered or it has rained. You could try washing the leaves with a very weak solution of household disinfectant followed by clear water & then drying the leaves. As bananas are a hot climate plant I suspect even an unheated greenhouse wouldn't be enough to protect it if you have suffer heavy frost. Saw something in a freebie magazine through the door today which might help protect it - fleece frost shields which are a fleece-type bags which you can put over vulnerable plants in their pots. They come in varying heights from 4 ft (�7.50) to 7 ft ( �9.99). Page 23 on the Presents for Men catalogue. Tel: 08701-203097. (These are expensive thought and you could probably make your own protection simply by buying a length of agricultural fleece from a garden centre and wrapping it gently round the plant. Don't use a polythene protection though as any condensation on it over a prolonged period will cause the crown on the plant to rot. If the plant is outdoors now, I'd get it under cover asap as the nights could suddenly get chilly. Perhaps somebody else who has grown one can offer more expert advice than this.