My head of department is going on maternity leave and I've been informally asked to take over in her absence. I'm keen to do it for my career development (practical experience to put on my CV), but I don't want to be working for nothing.
And so you shouldn't. The Government has been changing and clarifying the rules over the past few years. If you are required to undertake work of a leadership role, then you must be paid for that work, even if it is the lowest amount on the new TLR2 scale. As this is a maternity leave cover, expect to be offered a temporary payment. If the job entails the same work, you should be paid the same amount as the substantive head of department.
Make sure all is documented and above board, don't do anything unofficially.