In reply to Dave the Rave:
Evening! For a small gap like that, you could try using black RTV silicone sealant — it’s flexible, waterproof, and sticks well to rubber. Clean the area thoroughly first, then apply a thin bead and let it cure properly. It won’t be a perfect fix, but it’s a lot better than Vaseline and should stop the leak unless the seal is under pressure. Worth a shot before replacing the whole thing.