I've been a landlord since 2007 and have increased rents slightly over the years, never more than 5% at a time, never 2 years in a row, always with at least 3 months notice and always still under market value.
Most of my tenants are long term and had no issue with this.
I always make sure gas checks are done, boilers/bathrooms/kitchens updated when necessary etc.
Costs for maintenance, repairs and replacements are much higher than they used to be, same with landlord insurance plus the extra EPC and Electrical safety requirements now in place.
I think I am fair, but also it is a business/our pension, not a charity.
OP, I would suggest the agent approaches the tenant with a £30 month increase. They have done well for it to be the first one in 5 years!