Slow to start if left for a while

[Jezzer37]

Not sure how else to title this but hopefully the details will make it clearer.
My 2019 custom 300 Leader Ecoblue runs and drives with no problem at all. however, a few months ago it was not used much as I was working nearby for several weeks.
When I started using it again it had developed an issue where it takes quite a while to start when left for any length of time.
To be clear, it has always started, very occasionally on the second turn of the key, it just takes quite a bit of cranking!
Once its running if I turn it off briefly, such as when the auto stop/start kicks in, it will start no problem at all, its just when its left.
It has had a new battery which made no difference, it has no codes showing that would account for it so I was at a loss.
I have a warranty (limited!) so put it into an injector specialist in Bristol who currently have it but they cannot find the cause.
They have had the injectors out and tested with no faults and they tell me the engine is OK. Not sure exactly what they have checked to ascertain this.
The problem I have now is that unless they find a fault I don't think the warranty company will pay out, leaving me on the hook for their bill.
However I cant work without the van so am losing money each day, a week so far.
Searching here and elsewhere my first suspicion was fuel running back when stopped buy they seem to have covered this.
Next would have been compression maybe, but they seem to be dismissing this too.
I am going to contact the warranty company to see what happens if they find no fault, but has anybody had a fault like this that they resolved?
What else can I ask the garage to check?
I am getting to the point where I may just have to pay the bill and live with the fault

[metalworker0]

i would be looking at the pump in the tank.

next time
turn ignition on
then bash tank with big rubber mallet

if it then starts .. you know its pump jamming

all the best.mark

[Radiofence]

Another test...turn ignition, pop bonnet, press valve on bleed ripple on low line from the tank, if you hear air first before diesel starts coming out, then you can suspect an air leak.