OP
people are asking about the details of the document and guarantor status which are important to be able to answer your questions.
Is the EA a management company with you an arms length landlord? Many management companies handle things like guarantors without the landlord ever getting involved. Have you checked your contract with the EA to see if this os the kind of relationship you have entered into?
Did you receive copies of the guarantor agreement (be that tenancy or separate agreement) 3 years ago with the guarantors name on? If not, why not? Have the tenants signed new leases every year and had all data updated and have you received copies of all this?
If you did and the names are different, you need to find out at what point the name changed to the unknown person and who approved this. Your own administration should be able to identify this.
You have already stated there are things you haven’t read, could it be you ignored or skimmed through without taking in the communications regarding any change or amendment to the guarantor? Did you assume at lease renewal that the information was the same as before?
Are you required to co-sign the guarantor and tenancy agreement or do the EA do that as your management company?
I find it strange given the regulation around EA and rentals that they would outright lie.
In terms of resolution through ombudsmen or court for the “lie” - if the guarantor whose name you have now is financially liable for the damage costs and their financial details were enough to pass the guarantor test then the only difference between guarantor A dealing with it or guarantor B is in their name. Both will be party to the same contractual agreement. Both will have liability to reimburse costs regardless of their name. If the contract has changed and you haven’t noticed that is on you.
If the EA hasn’t done proper checks and guarantor B is in no position to pay for damages or if the agreement guarantor B has signed is different to the one you were aware of with A then you may (only may) have a negligence case against the EA but this would depend on a huge number of variables and would rely on you having saved all the evidence of prior communications and being able to show with your evidence that the EA was negligent in doing their job. That can be very difficult and risky and could end up costing you far more than the cost of repairs.
If you do not have copies of all your communications then you can ask for them through a subject access request. I think you need to be 100% sure at no time were you made aware of the change. This may not be explicit, it may have changed with contract renewal and you haven’t fully read it. That would be a poor show on the part of the EA for not making you aware but ultimately in that scenario the responsibility to read the
Contracts in full would be yours.