The fix

This is a very interesting post by Martyn Plummer on the JAGS News blog, describing how apparently silly details may make a world of difference. I think Martyn says he's now fixed the issue (basically, it appears that JAGS was sensitive to the order in which the model was written, eg at compilation you may have a staggering difference $-$ 16 minute vs 8 seconds $-$ depending on whether you defined the deterministic relationships between parameters first or last).

I'm writing this post mostly as a signpost for myself $-$ I guess you always encounter issues like this, which seem trivial, and the fix is so easy $-$ if only you had a bunch of little workers at your disposal all the time...