With the car developing a random heavy misfire, diagnostics predictably pointed to an injector. These are basically service items on this generation of 1.6 16v - they eventually get fried by the "excellent" Marelli engine management system which is prone to voltage irregularities (I was told once by an expert). Last one went about 12 years ago, according to the file.
Luckily genuine injectors are still readily available (wonder why?!) and only about 30 quid a pop, so I've replaced all four and we're back to living the dream. It's a bit of a fiddle because they are behind the front of the long inlet manifold, all of which has to come off.
Hopefully good for another 12 years....!