thanks for your replies, gentlemen.

I only use a water stop, so no fizzing noticed. pH sticks or strips might indeed be handy, so perhaps I'll head off to eBay.

I did also wonder about the pH being the culprit but on the other hand, I have never heard of unexposed emulsions being completely developed in this way, regardless of pH.

[On the other hand again, I managed to shoot film for the best part of 35 years without picking up a developing tank, and I'm only about 18 months into this, so my not having heard of something doesn't really count ]

I might give it another go, but instead of using my homebrew metaborate, stick to mixing weighed quantities of Borax and Sodium Hydroxide; the main reason for not having done so anyway is because the moment I take the top off my hydroxide jar and weigh it out, I can almost see it sucking up atmospheric water and I was concerned that this would throw off the proportions.

(That's why I started another metaborate thread here in the hope of getting useful some answers; unfortunately all I got was the usual "why bother/don't bother" responses, followed by the thread mutating into yet another abstruse disquisition on 4-mol vs 8-mol versions of borates.)