This could be a gearbox problem as it occurs when you're in full auto mode. I understand that a fluid change on these boxes is needed to stop problems happening. Not sure on how often it's supposed to be changed, but would guess yours is well overdue.

Hopefully someone with more knowledge of this will be along soon.

Thanks. Good idea re gearbox. I changed the CVT fluid about 5 years ago. Seemed slightly smoother to drive afterwards if IIRC. I am wondering if there is a "sensor" or other electrical link between the gearbox and the engine ECU(?) that could be causing the problem.

I maybe wrong, but after the 50k fluid change it should be changed at 37k miles, as for sensors not sure if this helps but gather the speed sensor can cause that under load as it happened to a friend

www.google.com/search?q=honda+jazz+cvt+gearbox+sen...1

Part is priced at between £116 and £128 each. Is there a way to test in situ before committingto this kind of repair by replacement? On a ramp I am guessing an hours labour to change both. I use a digi multimeter , but my electrical knowledge is sadly very basic. Thanks.

FWIW, I changed the CVT fluid. No change. Then one day the car had a sustained misfire rather that the momentary stumble. I did some more research on line and found this. I removed both banks of plugs ( rear were, as expected , very sooty ) and renewed, but with gaps set to .8mm rather than the 1.1mm specified. Car has run like a train since. Pulls hard in any gearbox setting and is now quite fun to drive...YMMV.