Replying to my own question! If at first you don’t succeed…
Going back into editing styles again, I realised that if using copy and paste, it is important not to paste apples on oranges…
- the conference paper reference type that I had copied from had used the field name Access Date - but the electronic article reference type has named the same field Date Accessed - as soon as I replaced Access Date with Date Accessed in the edited style the correct date appeared in the citation. Yippee! Sorry to waste the forum space - but who knows? - this may assist someone else.