An awkward scenario

Due to the numbers of peregrines in London, it is inevitable that they will find structures and buildings that are not suitable, either to hold territory on and ultimately attempt to breed on.
2 years ago I located an adult Tiercel on a structure, which due to its location and position always meant that, for breeding purposes, it was not ideal.
Early last year an immature Falcon turned up and the Tiercel was a happy boy, as expected due to her immaturity they did not try and breed but summered.Pre Christmas 2011, she now had her adult plumage, she disappeared for a while, some Falcons do this if there is no external threat from other peregrine pairs, food shortage has never been an issue in London. Late January she showed up again, the markings showed her to be the same Falcon, consequently nature took over and breeding was in the air with the pair copulating often.
Around March 28th she disappeared from view, consequent watching showed a couple of nest reliefs on the structure, by now I had decided exactly where she had made her scrape, it could not be seen but she emerged from the same position on 3 occasions with the Tiercel taking over incubation.
On April 8th after a series of heavy downpours, both the Tiercel and Falcon were permanently on show, obviously they had failed, the likely reasons being chilled eggs, lack of drainage on the level of the scrape or eggs simply rolling.
It is not uncommon for 1st time breeders to fail, in fact it is the norm but it does bring forth thoughts as to how to make them succeed.
Firstly every other tallish building in the vicinity is not ideal, either due to rooftops being used for Mobile Phone masts, they have to have 24/7 access, a nestbox and the Schedule 1 laws would stop any access. Others are just too low, additionally neither bird has never, as far as I am aware, used any of the surrounding buildings.
Secondly they never even looked at any other building/structure as an alternative; it was always going be ‘the structure’ where they are resident.
Working on the assumption that they will now hold territory every year and attempt to breed on this structure, it is likely that they will fail more often than not, this is usually the case in an urban environment unless they are assisted- Tray or Nestbox.Bearing in mind that the structure is not ideal for fledged juveniles and they may well ground, do you encourage them to successfully breed by placing a nestbox aloft knowing that it may lead to problems for the juveniles?
In my mind it has to be yes, obviously with the owner’s consent, they deserve the chance. Our unstable April and early May weather puts paid to many pairs on pylons, sometimes due to the fact that they don’t always use a stick nest and may well just lay on bare metal or some accumulated dirt. Consequently eggs become chilled/roll very easily, in short it is the exposed position that puts paid to them, hence the need for a nestbox.Such is the case with this structure.

A nestbox will solve the problem of a number of issues relating to their failure but successful breeding will also provide the challenge of grounded juveniles. In this case due to its very high urban location (people) and the dangers of, basically being run over if they ground, the only way I see forward is to publicise the site.
This will make the public aware of them and signs will have to be put in position during the breeding season, they are already known locally, this is the only way to safeguard them, as well as this local people can then get involved in their future.

It appears from the stats on the previous post  that  Falconers have been having a look at the hybrid, if anyone could shed any light on its identity it would be welcome.I have stated that it is an escaped Falconers bird as it fits no description, especially colour, of any Falcon that I know, if you know different I would welcome a comment.