I know - I can't help but think that I should have taken them and tried to hatch / rear them at home, but I know they would have had even less of a fighting chance if I had! So frustrating.That is what I worried might happen.
The nest was in danger from preditors anyway, but of course the biggest threat was if it was seen by humans.
People often think they are 'doing the right thing' when phoning the RSPCA, and walk away feeling good, not realising that in fact they are signing the bird's death warrant in some instances.
Maybe in this case someone did do that, but let's hope the adults are ok and will learn not return to that spot. Thank goodness the eggs hadn't hatched or we would be worrying about the demise of the babies.
You did the best you could in watching out for them, and thanks you so much for that.
I've learned not to call the RSPCA out any more, unless I absolutely have to, they're pretty useless. I know they're short staffed but (IMHO) they just don't seem to care - in the past they have taken more than 2 days to attend a duck rescue, when the bird had a broken wing and was trapped down a weir. I ended up fishing him out myself and took him to a wildfowl sanctuary - I know the RSPCA would have just put him down.
Anyway. I'm going to keep an eye out for the adults, and if they construct a nest there again, move it before they lay eggs.
Thanks everyone for your advice!