I've convinced myself there was a bit too much oil put in the car when it was serviced. When it first happened I checked several times to be sure and the oil was /right/ to the top of the top of the plastic maximum marker.
After about 3000 kms the average economy readings have improved - and I don't think that's just due to the trip computer settling down because I reset it with every fill and it shows -- until some data comes in.
Cleaning the accumulated gunk off the MAP sensors definitely helped based on seat-of-the-pants, MAFs not really, and spraying decarboniser into the intakes /possibly/ improved things but I couldn't be sure.
So I think it was really just a case of burning off a bit of excess oil and cleaning the sensors. Touch wood back to normal programming.
Thanks for all the thoughts and input, I learnt a bit in the process and got further under the bonnet than before. Cheers

