Guarantor Consent Required for Rent Increase?

Does a guarantor have to agree in writing to a rent increase?  
 
We are planning on serving a rent increase notice but wondered if the guarantor should   
a) agree in writing (for example, through a notification letter sent for agreeing and signing electronically)   
b) just be notified by the landlord that it is going to increase or   
c) it is not required

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