Citroen Xsara Diesel Engine 'Knocking' noise?

After having an alternator and the cylinder heaters replaced at my local garage recently, my Citroen Xsara diesel estate developed a loud 'knocking' noise and a slight reduction in power.

I took it back to the garage who did the earlier repairs and they said that the new fault wasn't related the the work they had done and suggested I had filled the car with 'bad fuel'

I agree that the engine does sound like a diesel running on unleaded fuel, but I know I hadn't done this, and I have had two fuel refills since the problem arose.

I took it back to the same garage for further investigation and they have now decided that the fault lies with the fuel pump and it would not pass an MoT because of emission problems.

Before I either 'write-off' the vehicle or spend money on a replacement pump I thought I would seek advise from this forum to see if those with knowledge concur with this view. The cost of a new pump I am advised would not be economic as the value of the vehicle is now less than £1000

The vehicle in question is a Citroen Xsara Diesel 1.9 Estate '52' Reg (no turbo).

Thanks