Hi thanks for the response. By waste unit do you mean what this diagram refers to as 'basket'? The products on Amazon seem to come with a 'fixed base' and 'basket', which slot in to the 'fixed ring'.
In my case, it looks like the 'basket' is integrated or is fixed as part of the 'fixed ring' - I don't know if any of these products would work or as you say I would be better replacing the full drain unit? I don't have much plumbing skills so was hoping I could go for a simple swap solution!