Hi, Rusty!
Here, "purpose -built" means built as such....so a flat would have originally been built as a flat.
"Conversions" are usually a different function of property then made into flats.
For instance, a large old house - probably in the "olden days" used for a large family, with servants' quarters (attics and cellars/ basements) - would be made ("converted") into separate individual flats.
Warehouses along the River Thames are being converted into penthouse apartments / loft flats /and luxury flat accommodation, and studios.
Does that help?
I have never heard "says" pronounced as "sez"?
But that may be what Mr. M means, as he queries why the word is pronounced as "saze"?
No, I don't put milk in my bovril drink...but then I drink my coffee black, too!
I don't know about visitation from the little folk....but since Mr Jingles left for your place, his twin brother seems to have moved in.....