Hi Jacko, the boat I am restoring had similar issues when I started, if the Hull is still attached to the inner frames then it should work out okay.
It will need you to strip the paint,back to the wood if you want a lasting repair.
Step 2 will be hard but well worth the effort, either using a rotary file/dremel type tool or a saw you will need to run it down the split about 3 to 4 mm deep.
Step 3 using a good 2pack epoxy like Bond Together from poundland, fill the slot with the glue making sure that its right down full depth.
Leave to fully cure for 24 hours then sand back to shape.
Step 4 coat the Hull in Ezekote resin and fine glass cloth. This will strengthen the whole hull. I usually use 4 coats of resin.
Step 5 using your usual methods you can now rub down, prime and paint.
Good luck, don't hesitate to ask if you need more help.