Rough rattling engine after DPF fluid refill (C4 1.6 HDI FAP)

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Last week I took my Citroen C4 1.6 HDI (110) to a garage to have the Eolys DPF fluid refilled and the counter reset. The car came back sounding terrible, with a horrible diesel rattle, and low power when driving in low gear. A couple of days later I drove it a short distance, less than one mile, then when I tried to start it half an hour later, it would start, then cut out after a couple of seconds. It did this repeatedly. I left it parked up and the next day it started fine and has been ok since, albeit still sounding rough and rattly. It seems to drive ok once at motorway speeds, aside from the engine noise. Additionally the average mpg, according to the computer, has dropped to about 40mpg (usually get 55) and similarly the distance to next fill showing on the fuel tank when full is 400miles (usually shows and achieves 700 or so). I’ve yet to see what mileage it actually does based in actual fuel consumed. This engine was always very quiet and smooth to drive right up until I took it in to the garage last week.

Any ideas what can cause this to happen, in general or, as in this case, literally either side of the FAP fluid refill? I suspect the engine is being starved of air (or maybe fuel?) but not sure where to start looking, maybe the DPF or EGR valve?

For a bit of background, the garage told me the DPF fluid container was completely empty when they filled it. That is plausible as when I had it checked previously the garage (a different garage) couldn’t reset the dpf counter/fault on the ECU as they didn’t have the right software. So the DPF fluid fill level warning has been out of sync with car’s counter (I understand the car counts doses delivered, rather than measuring the actual quantity of fluid in the tank) for a while. My best guess is that the car could have done maybe 2,500 miles without any fluid in it as it’s been used very little recently due to working mainly from home. Long story short, it seems unlikely that this isn’t related to something the garage did, but I suppose I can’t rule out a purely coincidental issue with the dpf due to uncertainty over the fluid level. In any case, as mentioned, the car had been driving and sounding perfectly normally (nice and quiet, responsive) immediately prior to going into the garage last week. Having reported the issue to them they’re saying it’s because there was no fluid in it and they’ve offered to do a diagnostic check on it (at my expense), but I’m a bit wary of taking it back in there.

There are no faults showing. I have used the Citroen/Peugeot FAP app to see if it shows up any problem. I don’t really know how to interpret the figures, but nothing looks too unusual to me. I’ve attached screenshots to this post (I think the red/green highlights just indicate rising/falling values?). The ‘deposits in FAP’ is reading 21g. I’m interested to know how this figure is measured, I assume there’s no sensor weighing the DFP, is there just a lookup based on pressure/flow? Also are the DPF reading’s given by the ECU reliable? Could there be an issue with the DPF and the readings still be low like this?
The car’s a 2012 model and done 160k miles. The miles are mainly motorway, I’ve used it less over the past couple of years but when I do commute it’s 40 mins on the motorway each way, so not lots of short trips or anything. Has been serviced annually, last service was Feb this year, oil and filters. Oil level is fine, oil is clean. As I said has always been quiet, smooth and great to drive, been a really reliable car for me and I’d really love to just get it back to normal!