Lol my advice is free
When was they're original contract supposed to end? And are they on a rolling month by month contract at the moment? If they should have already moved out but you let them stay as a gesture of good will they are still liable for the full amount of rent regardless if they move out on the 10th or 25th. If assuming some form of agreement was signed between both parties when the extension was granted then they are actually in breach of contract by not paying the full amount of rent.
Now to the deposit situation... also working on another assumption here - is the deposit held by the government? If so, only when you agree on how much of the deposit should be released and when it should be released will the deposit be released. If an inventory was conducted before they moved in and a check in the day they moved in there should be a list of any defects or flaws with the property that you have recognised. If this chip in the hob isn't included in that then you have rights to claim some or all of their deposit to repair the damage. However, I would warn you that if you intend to keep all of the deposit then the tenanst will probably go through the disputes service and try to claim some of it back so it may be worth seeing if you can replace a part of the hob for a portion of the deposit rather than it being held up in a dispute case for a long period of time. I would seriously consider getting an outside company to come in and do the inventory so that their findings are completely impartial. Also your tenants shuld be paying for a check out of the property which would also highlight the damage to the hob and any other damages in their time in the property.
Also, did you conduct any inspections on the property during their tenancy? If so, were the inspections documented with some type of form possibly including pictures? That should also back up your claim that the damage was caused whilst the tenants were in the property.
If I were you I would start getting quotes for the hob and see how much it would take to repair. Also double check your copy of the signed tenancy agreement to make sure that it covers you in the event of any damage to the property. One hting worth checking might be to see if your tenanst have accidental damage insurance. If so, it may be cheaper for them to claim on that to have it repaired and then no disputes need be raised over the deposit.
Hope that makes sense and helps! xxx