These new Regulations require landlords to have the electrical installations in their properties inspected and tested by a person who is qualified and competent, at least every 5 years.
He is ripping you off. They can write that at their discretion.
We have been doing these for decades in HMO’s. It’s every 5 years not at change of tenancy.
Report him to his governing body. Tell the body that if the tenant leaves in a month that period is ridiculous.
Next time ask before you book the electrician so you don’t get scammed.