Raising the rent the lawful way: Form 4A, notice periods, and the tribunal
With fixed terms gone for new tenancies, the statutory notice route is how rent moves. Get the form and the notice period right and it's routine; get either wrong and the increase simply isn't binding.
The mechanism
You propose the new rent on the prescribed form (Form 4A, for private assured tenancies in England). The new rent must start on the first day of a rent period, at least two months after you serve the notice, and no sooner than 52 weeks after the tenancy began. If the rent has gone up since, it must also be no sooner than 52 weeks after the last increase took effect (53 weeks in some cases: see Form 4A, note A3). So rent can rise at most once a year. Use the current form from GOV.UK on the day you serve, or let the calculator work out the date.
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If the tenant challenges
A tenant who thinks the proposed rent is above market can refer it to the First-tier Tribunal before the increase date. The tribunal sets the rent it considers the property would command, which can be lower than you proposed. An increase made the wrong way (wrong form, short notice) is not binding at all.
The tenant's application must reach the tribunal before the start date in your notice. The tribunal can set a rent higher or lower than you proposed, but the tenant won't have to pay more than the rent in your notice, and needn't pay the new rent until the tribunal decides. A tenant can also ask the tribunal to decide the open-market rent in the first 6 months of a tenancy.
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The practical rhythm
Diary the earliest next-increase date when you serve one, keep a copy of the served form with its date, and never rely on an emailed "rent goes up next month": the form is the increase.
Serve it in a way you can prove: by a method your written agreement allows or, if it names none, by hand, by leaving it at the tenant's address, or by registered post. Don't change the form's wording unless the form says you can: a changed form may not be valid.