Put some BP Ultimate, Shell V-power and give it a Italian Tune-up over 10miles or so.

" The car might be constipated and needing a blow through"

'Put some high octane fuel in an engine that doesn't require it and give it a good thrashing'

What a useless suggestion.

My car has lived on a diet of the cheapest supermarket swill for 15 years and the injectors haven't given any problems.

OP: Why are you measuring fuel trim? The important question is whether the air to fuel ratio is correct for all cylinders and fuel trim data isn't going to provide that. I'm not a gasoline injection expert by any means but I'd expect the fuel trim to be quite small. I'd expect the lambda sensor to only have the 'authority' to change the fuelling by +/-10%, to prevent a faulty lambda sensor from dictating a nonsense fuel curve that might damage the engine.

The lumpy idle is probably due to a lean mixture at idle. Does the exhaust spit or pop slightly at irregular intervals. Lean mixturs tend to cause this. Rich mixtures tend to give a softer exhaust note than lean ones. How does the engine cope with transients? Does it stumble when you snap the throttle open?

Have a look at your spark plugs. Are they all the same colour, preferably light brown? If one of them is a different colour, then the problem probably lies wit hthat injector rather than the rail pressure.

Quiet exhaust, black plugs, good throttle response-sounds like you're well on the rich side! If it is rich, I don't understand why the ECU is adding more fuel...

It's worthwhile noting, that the ECU doesn't 'know' what each cylinder is doing-the ECU assumes that each injector will flow fuel at the same rate and will set tyhe same pulse width for every cylinder. Due to manufacturing tolerences this is not exactly true, and the ECU will use the lambda sensor to fine tune the fuelling.

If the lambda sensor keeps 'seeing oxygen' in the exhaust, from say one lean cylinder, the ECU will keep adding fuel to ALL the cylinders until the oxygen in the exhaust goes away-this would result in the lean cylinder becoming stoichiometric-and all the other cylinders becomming rich. This is why I asked if the plugs were all the same colour.