"Buy some black Sikaflex and simply clean the old sealant surface/channel with white spirit, apply masking tape to the sides of the channel and gun the sealant down the middle. Get some water mixed 50/50 with washing up liquid and dip your finger in, then run your finger along the new sealant to smooth it, pull off the tape and you're fixed. Any excess can be removed with white spirit."
Sounds like a plan, thank you.