It is possible that the nest was abandoned early on and then discovered by a pair that started nesting nearby later in the spring, and these 'stole' all of the nest material for their own nest.
Although it sounds like this nest was clearly long abandoned, it is perhaps worth pointing out that it is technically against the law to clear out nest boxes before 1st September (this is to try and ensure that no-one 'cleans out' a nest box that is being used by birds that are trying to squeeze in a late brood towards the end of the summer).
RoyW it is perhaps worth pointing out that it is technically against the law to clear out nest boxes before 1st September
I think that the RSPB definitely need to amend this.
Even though it is very unlikely that any harm would be done if nest boxes are cleared out in August, it is still against the law to remove unhatched eggs at this time.
It is actually illegal to remove eggs from nests at any time without a licence, but the removal of unhatched eggs is covered by the general licence in this link:
Perhaps any RSPB staff member who reads this thread can get in contact with the relevant person(s) and get the relevant RSPB web pages changed?
Hello, thanks for pointing the error out on our web site, I shall pass the details onto the web team.