Thanks for the advice. The cleat that had broken was a white plastic one, which didn't look like an original fitting. I'm hoping that it was something to do with the way it was fitted which has caused the corrosion. To fit the replacement, would you suggest aluminium rivets, stainless self tapping screws or something else?
Something else. Monel pop rivets.
The reason the old one corroded out is the proximity of two different metals - stainless steel and aluminum - which in the presence of salt water makes a battery. The aluminum loses.
If you sail in salt water, you need to rinse the boat (all of it) with fresh water regularly to get rid of the electrolyte (salt water).
My suspicion is that that mast leaked, trapping salt water inside. Very bad in the long term. That's why I think you're going to have other problems - the worst of which is going to be a mast base that comes off when you're stepping the mast.
While it's tempting to go back with aluminum pop rivets, they aren't strong enough. Monel is less reactive than stainless steel, but more expensive.