thanks Aris - no the curd didn't smell - the ph meter is reliable and I checked its reading with a ph strip - it was cleaned and calibrated before use. My explanation is that I used a bit more culture than I might have .9G for 10 l milk and that the curd which had been matured for 90 minutes before cutting, was then after cutting kept at 31C for another 90 minutes before being put in a warm cloth lined colander for three hours. It gave the culture a massive boost. I supposed I wasn't surprised that the ph dropped fast but it didn't give it enough time to harden once drained so when I salted it , it was still quite wet and friable. It's draining well and it looks fine so it will probably be OK.
Mysteries of cheese making !