That was going to be my next question!
Cheers Martyn.
Chris.
Yeah, superglue sounds a bit Heath Robinson, but it's used by many of the top custom makers. It makes for a very tough and durable finish.
For filling in cracks like this, you will need the really runny/watery stuff (a lot of superglues have added thickener to make them more manageable, avoid this type). Drizzle a little on the crack and let it "wick" into the crack. Let it dry - if necessary, smooth out any runs while it's still wet by rubbing with a finger covered in clingfilm (this minimizes sanding off). Build it up using several thin coats with a light sanding in between each coat. Keep adding superglue untill the cracks are fully filled and there is some glue overlapping the area filled. Then
gently sand back down using progressively finer grades of wet and dry paper. You are not trying to re-shape the handle, just take the glue down to a fine and flush finish. When the glue-fill is flush with the wood, use the finest grade paper, followed by a buff with some wax polish. The crack may be visible still, but it'll be well sealed, it'll look quite nice, you wont be able to feel it and the finish will be tough and durable. You can coat the whole handle if you wish which would give an almost invisible mend, but go easy as it's possible to botch it up.