It's classed as a consumable item as they don't repair well, you'll need a new one. However, the use of silicone sealant and gaffer tape has been known to extend bodyshell life!
If like me you are a tight ass then this is what I did. I drilled 1mm holes about 5mm either side of the crack and then stapled through those holes with normal staples. Then get some pliers and squash the staples flat and cover them with tape. will last for ages and costs nothing!
I should have a new one coming but I want to make this one last as long as possible before I use the new one, could you help me with the on Tuesday please ian?