The car would give a mil with P0251 if the was a leak in the fuel rail or drop in pressure.
However, it would need to see 3 instances in quick succession to illuminate the mil.

think I'll just fit a Thule roofbox and fill with a big plastic bag of water with a pipe running to the engine, and a foot-operated pump like my cortina used to have for washers.
Anybody else done this yet? Any tips?

So does anyone know what the software update does?

Had the little hiccup again today on the way home, low revs 30mph just going really easily in traffic. Feels like loss of power for a fraction of a second then immediately OK. Never done it until the day I had the work done. Coincidence? Problem is it is so infrequent it will be a pain to find I fear. Twice Tuesday evening when I got it back and once tonight. Other than that the car is running brilliantly.

Mine seems to have a "software update" every time it goes in the dealer but still "hiccups".
Usually during warm up but can happen at anytime.

The hesitation at low revs until the turbo kicks in is likely to be due to the Inlet valve sealing issue caused by the inherently faulty EGR - assuming this is 1969cc VEA D4 or other version of this engine. Mine was exactly similar and a dealer correctly predicted the outcome prior to leakage tests. Repairs including blocked (again) EGR cooler and Inlet manifold for the second time and all inlet valves cost £4,000. Eventually 95% paid by Volvo. See other posts on the same subject of Volvo issuing a belate4d safety recall for what is merely the latest manifestation of issues caused by the inherently unfit for purpose EGR system.

You (Clan) obviously know about this subject but presumably accept that the cooler fix didn't stop continuing problems to a significant number of cars. Certainly the cooler piping etc fix seemed to keep the EGR valve area cleaner for longer, but deposits occurred further down the system. See posts in this section and on the EGR thread itself about the typical hesitation / hiccupping indicating loss of inlet valve seal. Also blocked inlet manifolds, coolers etc as well as new valves - coming to £4K in my case.
The recent BMW issues of similarly melting manifolds and fires were an EGR issue related to coolant contamination getting into the air flow - whether this is the case with VEA engines or its a combination of blockages and combustion "blow back" etc will be known to Volvo. Either way they clearly wish to disguise the fact that the culmination in a safety recall is actually due to the EGR system (not just the valve) defect they have been dodging for some years. Would you advise spending £30K or more on a 2019 car with a potentially deep seated EGR problem and no proposed (let alone guaranteed) fix?

Report EGR faults to Honest John
 

This thread and the EGR Check light thread are essentially about the same thing, which is the long running issue of EGR system faults affecting a significant number of VEA 1969cc diesel engined (commonly D4 but others too possibly) cars. The faults have continued and developed into seriously expensive issues AFTER the circa 2016 "permanent fix" which apparently stopped the floods of EGR valve blockages but allowed bigger problems to continue
Typical faults on all threads and I am assured as presented to dealers go from warning lights and sensors etc to include blocked coolers, inlet manifolds and engine "hesitation" caused by inlet valve obstruction. The current Inlet Manifold fire risk recall is clearly also EGR related as other than air the only materials entering it come via the EGR. BMWs recent identical fire risk symptoms were EGR related. Any fix around the EGR will highlight the whole range of expensive non safety issues, which Volvo have been dodging, presumably hoping passage of time and eventual premature scrapping would make go away. Going by the almost £4,000 paid to me they must have spent millions in "goodwill" to pacify complainers and avoid admitting failure of the "contractually binding" permanent fix promise. By their own figures that's affecting potentially 70,000 cars in the UK alone.....
After exchange of emails with Honest John (of his award winning motoring website) he has already put some of the key EGR questions to Volvo. Not least is around the position of those buying or just bought 2019 cars... Unlike some other publications he doesn't just regurgitate makers PR, for example his writing on VW 1.5 TSi petrol engine current issues
He accepts there are loads of online EGR posts including on his own website, but to officially evaluate evidence of any underlying issue on Volvo's part asks for details of car, owner and post fix fault to be emailed to "letters@honestjohn.co.uk". He requests 200 words max. If Volvo have been less than honest with their customers he may be well placed to find out....I suggest it is in the interests of all concerned to do so.

So given I do have a 2015 D4 and it has just had the manifold replaced and that it also has developed this very occasional hiccup at low revs, am I like to encounter more issues in the near future?

Although I use the car for work (6 mile journey) it does get a long distance run every tank of fuel/month as I go out places to do some photography throughout the year.

Should I contact the dealer again and report this low revs hiccup that I never had before the work was done?

The recall notification letter implied there was no technical fix at the moment, and they would be in touch later. When I asked the dealer if they were going to simply replace the parts with like ones that were faulty because there was no technical solution he simply replied "This is the fix.

It's officially now a 'Recall' as my car on the Gov MOT site is marked as subject of a Recall since 2nd August.
Pop your reg. no. in and all will be revealed!

Outstanding recall found
This vehicle has been recalled since at least 2 August 2019.

Contact a VOLVO dealership to arrange for repairs.

If you've had the recalled component repaired recently, it can take up to 3 weeks for the manufacturer to update their records.

Knowing my luck the recall will be for the work that I already had done last week, and this low rev hesitation it has developed since will be part of the EGR valve issue that probably won't be getting done.