I don't know about the Jazz system of that era, but some EGR diagnostic systems use a temperature sensor to decide if the EGR is actually flowing gas...
Interesting, I couldn't seem to pin down how they check themselves.
Anyway I took the aluminium blanking plate off and there was a clear mark, almost a dent, where the valve had been hitting it. Bit annoyed with the plate seller who should have known that and should have done what I have now done..
Which is to drill a large hole in the plate, bigger than the valve, so it can open all it likes into the exhaust but any gas it lets in won't be going anywhere as the air intake port remains blocked. I had to fit a new gasket between the plate and the valve (with the old one still between the plate and the iron exhaust circulation body.)
I had assumed that the EGR was throwing codes because it was hitting the plate (i.e. the "valve displacement/position measurement" you mention) but it may be the temperature thing. Either way that should fix it. Long journey tomorrow, if I don't say otherwise assume it did fix it.