Thanks for the help and advice - spent the past days here trying to get rid of photos on my iPhone 6 and boy, do Apple make a real mess of handling devices what with the move from iPhoto to Fotos etc. A long way away from simplicity and user friendly which is what Apple are supposed to symbolize. Anyhow, I had to:
1. Deactivate all cloud supporting preferences and make generate an empty directory for synchronisation
2. Still with approx. 5000 photos actually on iPhone device, I had to start iPhotos, set-up preferences to open when iPhone is connected
3. Then ensured the 5000 photos were actually in the Foto library (so not deleting non-stored photos)
4. Using Image Capture (stupidly named Digitale Bilder in German - why who knows!!!), select all using cmd-A and then pressing delete (also not easy to guess with the circle with cut -through) - this did the job
5. Reenable iCloud for future storage.
Starting to hate Apple after being a fan for nearly 15 years!!!! Please simplify Dear Apple developers for the future.
Cheers!