Do bailiffs just hammer on the door for a while and then leave?

Yes

If nobody answers the door, the bailiff is achieving very little for himself standing outside your property.

They will eventually move on to their next address.

Do not open the door to unexpected callers, or to anyone dressed in body armour and bovver boots. It usually means they want money.

There is no provision for bailiffs to break and enter domestic property unless the bailiff is recovering unpaid tax owed to HMRC and even then it is rare due to the high risk of damages.