Boiler banging and dead system

Help. I have an Appollo boiler and woke up this morning to it banging very loudly, then shutting down, then repeating. It cyles about every 20 seconds. When it fires it sounds like it's nearly empty, I can hear what sounds like air going through the outfeed pipe. The pump is not vibrating and although it seems to have got noisier of late is making no noise. I have not checked the header tank yet but it was OK about a week ago (I also noticed in the header tank at that time a silvery-grey film floating on the water).

Im guessing the pump is dead so Ill buy another and replace it. I have several questions:

What is the film and did it have anything to do with the increasing noise of the pump and its demise?

How do I seal the pump connections, do I need ptfe tape or are there gaskets involved?

Why would the boiler sound so empty, should it have filled up again from the header tank even though the pump seems dead?

Thanks in advance for any help you can give.

........and although it seems to have got noisier of late is making no noise.

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Well? Is it making a noise, or is it making no noise?

Don't just change the pump, this is not the way to fix a problem (unless you are a professional )

Just because the tank is full of water it doesn't mean the water is getting to the system. Drain some water from the system, and see if the tank fills again. If it doesn't there is a blockage in the tank outlet pipe. (Very common)

new pump will come supplied with washers

boiler wont be empty but if pump has failed, boiler will crash & bang due to low water content heat exchanger

film on top of water in header tank. I have seen before and im sure guy from fernox told me is caused by bacteria from dead insects floating on water

Sorry about the confusing post oilman, I had to hurriedly edit the original as for some odd reason the apostrophes in the text drove it mental. I must have confused myself in the editing.

I've sorted it. I got back from the office to confirm the pump hummed but didn't turn so I replaced it. I also drained the system and scooped out the scum and loads of a brown mud-like substance from the header tank, flushed the whole lot through and then refilled with water and the fernox-type stuff. It took a while to fill the boiler and run the air out, it seemed almost empty. I finished off by bleeding the rads and it's been running sweet since, in fact it's the first time the system has run silently since I moved here 2 years ago.

Thank you both for your posts.