Thanks for the info. In the end I figured I might as well jump to v5.3.1 seeing as I had to do a bunch of verification either way. That did solve it; it turns out the i2c driver was completely refactored in between. One thing did catch us out - we started to get timeout errors on the bus. On digging...

Just a quick post to save others wasting time.
I got an email from Espressif suggesting it might be a known issue in v5.1.2 that was fixed in v5.2; I'm going to update and get back to you.

I'm trying to use I2C 0 at a slow 5kHz baud rate, but I'm finding that I only get a few transmissions at the correct baud rate and then it suddenly increases to 40kHz for the remainder. To try and track it down I put this code at the top of app_main() - the rest of my code is still in the build but ...