drum is filling with dirty water overnight

No, you should just be able to insert the connector in the end of the current drain hose and connect the new extension to the other end. You need to make sure they are connected properly so it doesn't leak. If they came apart it would flood your kitchen.

Ok I’ll order an extension kit - is there anything I need to know before trying to fit it myself? Besides turning off the power to the washer, do I have to turn off the water or something because I don’t know where to do that

Hi. The hook is for use when the drain hose is plumbed in to a pipe with a u-bend at the bottom. You don’t need it when plumbed into the sink u-bend.

Not sure what the other pipe is for but you need a proper connector to extend the drain hose. Also it looks too thin.

These came with the washing machine, can I use any of them to extend the drain hose at all?

Thank you so much!

Here are some examples. The spigot that the drain hose attaches to needs to be pointing straight up. The drain hose that attaches to it needs to point up – before pointing down. This stops any water that runs down the sink from getting inside the drain hose.

In your case, I would use a cable tie, and fasten the drain hose to the white plastic pipe above it to prevent it falling down. But as I initially said, you will need a lot more drain hose to be able to do this, so if you can't pull enough through you will need to replace it with a longer one (2.5 metres probably),or fit a drain hose extension kit.