Go somewhere else. The EGR valve is only a valve, and like any other valve only opens and closes. It does not perform any other magic function. If it opens and closes and is clean and clear then there is nothing else that can be wrong with it. What can and probably is wrong is the inlet manifold is blocked with carbon deposits which is restricting the inlet air flow. This would be even more likely if this is a low mileage gently driven car. Modern diesels are not suitable for town driving. The EGR valve could be stuck open which would mean too much exhaust gas in the air intake. The engine would be choked of oxygen and the symptom would be anywhere between the engine running very poorly to not running at all.

Glow plugs are heating elements and are very easy to test. Failed glow plugs may prevent DPF regeneration (if your car has one) which could also be a cause of your problem.

The alternator is the main component in the vehicle charging system. A simple charging system test will tell if it's working or not.

Asked them would alternator cause juddering and smoking of exhaust. Garage - Yes

How? If the alternator is not charging then the answer is no. If the casing is broken and it's hanging on its mountings then I suppose it would cause a vibration, but it would definitely not cause smoke in the exhaust. How on earth could it?

I suppose you could authorise the garage to replace the alternator on the clear understanding that if it does not solve the problem (which I suspect it won't) you will not pay for it. That may force them to look at it differently. It could also be the garage has caused a problem whilst looking at it, and are looking to you to meet the cost.

Do you think they could temporarily get the engine running to diagnose the possible problems you have mentioned, without replacing the alternator.

Its is a diesel car.

I can't see why not as long as the battery is fully charged. The ECM will need to see a minimum battery voltage, but if the engine doesn't need to be running the garage could hook up a battery charger to reach that voltage. That's pretty much standard practice anyway.

The way I see this.is that you have a 10 year old diesel Corsa fitted with the Fiat multijet engine. These engines are not robust and realistically probably coming to the end of its useful service life. I would be inclined to not spend any further money on this , move it on ,and get a petrol engine car next time.

I agree - I ran a new one for 50,000 miles as it suited my purpose at the time.

Get rid of the thing as soon as you can.

A total nightmare of an engine, caused me endless trouble and expense - part-exed it for a Honda Civic - no further problems.