the purpose of main bonding is to ensure that all the metallic services coming into the house are at the same potential as the house electrical earth.
For example, if you have an electric kettle, it will be connected to the earth wire of the electrical system through the earth pin of the plug. If you have the kettle in one hand, and the cold tap in the other, the bonding should ensure that you cannot get a shock even if the kettle is faulty, because there will be no voltage between them. Even in the very unlikely event of the water or gas pipe becoming live, for example if a tree is blown over and a power line falls on a gas pipe outside, you should not get a shock even if you touch e.g. the gas boiler or cooker at the same time as the electric kettle or a metal light switch.
So it is a basic and important safety feature that has been required in electrical installations for many years.
AFAIK there is no legal requirement for landlords to have electrical safety regularly inspected. However a good ploy is to write a letter (not make a phone call) to the agent saying that an electrician friend commented that he could not see any evidence of main bonding on the gas and water services, and can they confirm that the electrical installation is safe and up to standard please. This tends to frighten them, because if they say it is, and you are later electrocuted, they will be in a difficult position. So there is a good chance that after they have tried to weasel out of it, you can get them to pay an electrician to inspect, test, and if necessary rectify it. Main bonding in a small flat is unlikely to be as much as half a day's work, it might only take an hour or so. If they send anyone, ask them to confirm that he is a qualified electrician who is a member of a self-certification scheme (although this is not strictly necessary, if they employ an unqualified handyman and he doesn't do a proper job, they will, again, be in a difficult position if you are later electrocuted). Your trick is to make sure they can't say they didn't know. It is, of course, very unlikely that you actually will be electrocuted, but it does no harm to make them observe elementary safety rules.