Hi Colin,
That's your charcoal canister

Part of the anti-polloution tackle fitted to petrol engines.
What it does is it stores petrol vapours from the tank, preventing them venting to the atmosphere when the filler cap is removed and the tank is being filled. Petrol vapours venting to the atmpshere resluts in the emission of very large concentrations of HCs (Hydrocarbons) into the atmosphere and is the worst form of automotive polloution. Charcoal canisters are good fairies as they do no performance harms whatsoever to the engine and actually do a lot of good

I approve of them!
When the engine runs, under ECU control, the vapours stored in the canister are sucked into the engine and burned. The brown device is the purge electrovalve and off-times woul can hear it buzzing as the ECU activates it to allow the fumes to be purged out.