So, printer 1 was printing black pages, then I put a toner/image drum from another printer in and still got black pages (which suggested problem with printer).
Then tried toner/drum from printer 1 in printer 2 and got all black pages (which suggests problem with toner/drum) and then when the working toner/drum returned to printer 2 in does not print text, just get streaks down whole page (which suggested problem with printer).
Toner/drum from printer 2 then tried in printer 3 and it prints OK, when put back in printer 2 it prints only streaks, no text. And toner/drum from printer 3 in printer 2 prints no text only streaks.(which suggested problem with printer).
As printer 2 was working and has the same toner and drum as before (and which print in another printer), it appears the toner/drum from printer 1 has 'infected' printer 2 in some way to stop it printing text.
Toner and image drum are genuine Dell parts
Any suggestions how to restore printers to working state?