Can I legally repair my own gas cooker or hob?

For most repairs the answer is no — you need a Gas Safe registered engineer. Under the Gas Safety (Installation and Use) Regulations 1998 it is illegal for anyone who is not on the Gas Safe Register to repair, service, install or even disconnect and reconnect a gas cooker, hob, oven or fire in the UK. That covers almost everything people mean by fixing it: a burner that will not light, a weak or yellow flame, a faulty flame-supervision device, or any smell of gas.
There is one narrow exception. The regulations let you replace or adjust the parts a manufacturer designed for the user to handle — a control knob, a pan support, an oven shelf or a light bulb. So if your won't-light problem turns out to be a blocked burner cap you can lift off and clean, or a flat battery in the spark ignition, you can safely sort those yourself. Anything that disturbs a gas-carrying part is off the table.
Two practical tips. First, before any engineer starts work, check their Gas Safe ID card and look up the licence number free on the Gas Safe Register — that check is your responsibility, and it is what makes the work legal. Second, get the fault diagnosed before you write the appliance off: many dead cookers need only a new igniter or thermocouple, which is far cheaper than a replacement. For typical prices, see our guide to gas appliance repair costs.
If it does need a professional, BestBuilders can match you with local gas engineers for free, no-obligation quotes — just run that Gas Safe ID check yourself before any work begins.