Hob crack - who is responsible for replacement - In England

The Situation:
I’m currently at the end of my tenancy and there is a very small crack on the edge of the ceramic/induction hob. It wasn’t there when we moved in, so while I’m not 100% sure how it happened, the letting agent is now demanding a full replacement.
The Issue:
The hob is approximately 5–7 years old. The agent is asking me to cover the cost of a brand-new replacement.
My Questions:

1. Am I legally obliged to pay for a new hob, or is this considered "betterment"?
2. Since the hob is nearing the end of its expected lifespan, can I request the landlord to contribute to the cost (apportionment)?
3. How do deposit protection schemes (like TDS or DPS) usually view a "tiny crack" on a 7-year-old appliance?