Wood Pigeon Fledglings, Parents left?

Hi everyone, I just joined here to hopefully find out from someone more experienced with Wood Pigeons about this.

Full story: (see bottom of post for short version)

I live in Plymouth, and work at Derriford Hospital, at work we have a nesting pair of Wood Pigeons who have made their nest in a very easy to view spot in the hospital!

It's up a tree in an enclosed courtyard, the nest is level with the windows of a waiting area on level 6 (ground level for one side of the hospital). It's a decent sized tree and being in the corner of an enclosed courtyard is nice and sheltered. The nest is 1-2 feet from the window so it's really easy to see, and me and my girlfriend who also works at the hospital have been following the parents progress the last year or so and have become quite fond of 'Helga' and 'Karl' as we have named the parents lol. We initially just called the bird seen on the nest Helga, but since learned that both the male and female take turns incubating the eggs and such so we had to name the male too 

The 1st nest, and 1st clutch of eggs were not sucessful sadly, they did hatch, but we think the parent(s) possibly got spooked from the numerous people coming and going in the waiting area and no doubt getting too close and they must have got too nervous or felt threatened and left, and the chicks died very young  Then we had that storm hit the UK and the nest got destroyed.

Fast forward to this spring... The parents came back and rebuilt the nest in the exact same spot (so must be the same mated pair we assume?).
This time the 2 eggs hatched and the chicks have been doing great and are now fledglings.

And here is where I would like some advise, help, reassurance, or all of the above! As I would hate for these fledglings not to survive after the parents did so well this time. Me and my girlfriend really care about them all and would be devastated if anything bad happened to them.

We have not seen either of the parents now though for a few days, perhaps 3-4 days now, it's hard to tell though since we cannot be watching them 24/7 as we're doing our jobs throughout the day, and go home when we finish. I do try to check on them as often as possible during my shift whenever I am passing through or on lunch etc.

I have a ton of photos and videos of the parents, and the fledglings. I would estimate their age to be between 20 - 23 days, approx. They have gotten alot mof of their adult plumage the last week or so, and have stopped using the nest as much to rest and are now using branches near the nest to roost either together or apart, they're always within a few meters of each other if not together.

I'm worried that if the parents have gotten scared again and left that they're not getting anymore feeds and therefore any food or hydration, how long will they be ok for if the parents have stopped coming and feeding? Neither me nor my girlfriend have seen the parents the last 4-ish days. It could be that the parents only now make flying visits (excuse the pun), to feed and are then off again and we are simply missing them each time? But it seems odd not seeing a glimpse of either of the parents in this time. Would they not come back to roost in the nest or surrounding tree branches still? Or do parents leave the young to fend for themselves around this age to encorage them to make their own way and leave the nest?

I thought I read somewhere that the parents and fledglings stay together for a time after they grow up as a family before they go their separate ways or is this wrong?

Any help would be amazing as I really want these fledglings to survive and have the best chance at life. The thought of them not making it now after how well they've done is really upsetting...

[I will add some photos as soon as I upload them to my computer from my mobile].

Sorry for the long rambling post!

TL;DR. 20-23 day old fledglings seem to have been left on their own by parents, will they be ok?