My 1.6tdcr diesel has an intermittent high engine idle speed.normal is 780 but every now and then.

it increase to 1000. Does not matter if engine is hot or cold.

My other problem is again intermittent with a very rough and lumpy idle speed either at the normal 780 speed or the increased 1000.

Both problems come and go as they please.

Car has 14k miles on and is returning average mpg of 50 on a variety of journeys.

Have not had it back to garage yet, any ideas before I book it in

I have noticed that the idle speed is higher if the car is still moving when the clutch is depressed, once the car is stationary then the idle drops. The rough idle could be when the dpf is in regen.

The fast idle will be the DPF regenerating. What kind of journeys do you do? A lot of short ones around town or long motorway trips? If doing a lot of short trips or slower speed trips then the DPF will fill quicker so will have it regen more frequently.

My Octavia 1.6TDI Does the same.

I do 28 mile trip to work (Motorway), and even after 18 or so miles it can be idling at 1000 RPM.