You silly sods!
Any hoo the chaps down at teh Volvo have reported an intermittant error for which I need to "keep an eye out for"
It's called "Long term fuel trim" and a bit of Googling reveals thus:
Feel free to nod off or leave half way through.....
1-1-3 Short Term Fuel Trim (Integrator), Upper Limit
Condition(s) for DTC
The short term fuel trim value is a measure of the manner in which the injection period included in the basic MFI module program must be varied to ensure that the engine is supplied with the optimum air/fuel mixture. This optimum mixture corresponds to a value of Lambda = 1. If the engine is running lean, the short term fuel trim will extend the injection period to restore the optimum mixture. A DTC will be recorded if the engine runs so lean that the short term fuel trim reaches its upper limit and remains there for 166 seconds.
Fault source(s)
Heated Oxygen Sensor (HO2S ) signal lead short-circuited to ground.
Heated Oxygen Sensor faulty.
Contact resistance and oxidation in Mass Air Flow (MAF) Sensor connector.
Mass Air Flow Sensor faulty.
2-3-2 Long Term Fuel Trim, Idling, Upper Limit
Condition(s) for DTCs
The MFI module receives information from the Heated Oxygen Sensor (HO2S ) if the air/fuel mixture is too lean in the idling range (2-3-2) or in the part-load range (2-2-1). In this case, the HO2S signal will usually be less than 0.5 Volts . The MFI module short term fuel trim compensates for this by enriching the mixture so to maintain a value of Lambda = 1 . Deviation of the short term fuel trim mid-setting from its normal value will be compensated by the long term fuel trim. Enrichment by the long term almost to a maximum (upper limit) will be interpreted as abnormal and the DTC will be recorded.
Fault source(s)
Open-circuit in HO2S heater supply or signal lead.
HO2S heater signal lead short-circuited to ground or supply.
Faulty HO2S.
Contact resistance and oxidation in HO2S connector.
Faulty Mass Air Flow (MAF) Sensor.
Low fuel pressure.
Air leakage.
Incorrect sensor signals.
5-1-2 Short Term Fuel Trim (Integrator), Lower Limit
Condition(s) for DTC
The short term fuel trim value is a measure of degree to which the injection period in the basic MFI module program must be varied to ensure that the engine is supplied with the optimum air/fuel mixture. This optimum mixture corresponds to a value of Lambda = 1 . If the engine is running rich, the short term fuel trim will reduce the injection period to restore the optimum mixture. A DTC will be recorded if the engine runs so rich that the short term fuel trim reaches its lower limit and remains there for 166 seconds .
Fault source(s)
Heated Oxygen Sensor (HO2S ) signal lead short-circuited to ground.
HO2S faulty.
Contact resistance and oxidation in Mass Air Flow (MAF) sensor connector.
MAF sensor faulty.
And I bought a Petrol car for it's 'simplicity'
my lack of effort to sell it has helped me out now! had a nice old waft to work this morning with the heated seat on 
)
'Ere Welly! Should we start an French Unsold Red Car forum 
), you can eliminate a lot of the guesswork from diagnosing a fault if you can find a reference of what values a given sensor should be reporting
