What can you put in a tenancy agreement?

I recently had a landlord who wanted to write his own tenancy agreement. Apparently he'd done it that way for years! Clearly this is a landlord's prerogative - it's their house at the end of the day, and many landlords operate without an agent using either a tenancy agreement they've written themselves, or no tenancy agreement at all. However it made me wonder why he'd chosen to use a lettings agent at all - it's a bit like me engaging an accountant because I don't have the expertise in a particular area that they do, and then not taking the advice I'm given and deciding I know best.

Unsurprisingly, the tenancy agreement he gave us was pretty poor, and showed a complete lack of understanding of what's legal and what isn't! When your property is rented by a limited company, you use a contractual tenancy agreement, which gives the landlord far greater flexibility to write whatever clauses he likes as long as both parties agree. When your tenant is an individual (99% of tenancies), you use an Assured Shorthold Tenancy (AST) which is governed by legislation, leaving far less flexibility for landlords to insert random clauses. Examples include:

1. The Housing Act says what notice periods must apply by law to an AST. The landlord can't simply overwrite this by getting the tenant to sign something.
2. The responsibility for arranging the annual gas safety certificate is the Landlords, not the tenants, irrespective of what the tenancy agreement says. You can't simply amend the law by mutual agreement!
3. The tenant has a legal right to the 'peaceful enjoyment' of the property, and this right carries more weight than the landlords right to inspect the property or visit the property. From a landlords perspective, this can be frustrating, but it's legally correct.
4. The tenancy deposit must be registered in accordance with the rules! Again, you can't include a clause that bypasses this requirement. 
5. Generally, and as with all contracts, there's such a thing as an unfair clause. For example you can't insist on the tenant sweeping the patio once a week! Just because you've got your tenant to sign acceptance of an item, if you end up in court over the matter later, a judge could still find in your tenants favour on the basis that the clause in the tenancy agreement was unfair. 

Surely one reason for using a lettings agent is to ensure that the tenancy is set up correctly, and as such any later problems are minimised? If you're using an experienced ARLA lettings agent, you'll find we all have slightly different tenancy agreements, but the clauses in them all come from:

1. our professional bodies, who update us on changes to legislation and
2. our experience of managing thousands of tenancies over the years. 

I started trading in 2006, and recently compared what was in our first tenancy agreement with what's in our tenancy agreement today. The two are very different! Whilst we're constrained by legislation to some extent, our standard tenancy agreement has evolved over the years, learning from experiences and mistakes, to protect our the landlord (our client) as much as the law allows. I have solicitors giving me examples of their "latest recommended" tenancy agreements which are never in plain English and, quite frankly, aren't a patch on ours when it comes to looking after landlords interests at a practical level. It's an example of an area where, although we're not legally qualified, we actually have more working knowledge than someone who is.