P0401 EGR insufficient flow fault

[Kath]

I have an Audi A4 2.0 tdi technik. A short while ago it came up with P04010 EGR insufficient flow detected error. We have cleaned the pipe from the block to inlet manifold (dirty but not blocked) and replaced the whole EGR/cooler unit (EGR valve, vacuum actuator and sensor all come as one unit) - the old cooler was blocked solid so the error code made sense.

we cleared the error code after replacing the unit but it keeps returning. Sometimes, but not always, it triggers the engine management light.

any ideas please?