Looks like you are correct about the blocked exhaust connection. The poor running and high fuel consumption started immediately after MOT a few weeks ago. Enormous lengths of hair-like wadding started dangling from the tailpipe of the rear exhaust box .. and I removed them .. also corroded metal inner baffles. This went on for several weeks, like a magician's hat spewing scarves .. then it stopped, presumably all released. The poor running and fuel consumption improved a little, but not back to normal. I had the exhaust system checked and it was externally sound and apparently normal flow pressure .. Kwik-Fit said leave it. It would surprise me if it was still blocked, but how can I check? Is it really necessary to change the expensive exhaust, even though I know the back box is now "empty" ?
Then I discovered the burst SAI air hose. Could this have been caused by back pressure during the period when the exhaust wadding was obstructing the flow? Crimping the air hose leak has helped, but not totally resolved the fuel consumption. Also, if this air system only affects the first 30 seconds of running, would the burst hose really explain the poor fuel consumption?
I will check those rubber couplings either side of the engine in daylight, thank you.