It's a funny thing - you can wait for ages for a Pilkington point to crop up on a development, then a number come along at once! It seems to be the "in point" for clients and planning consultants at the moment, perhaps as a result of development options for already permitted sites being refreshed in the current climate.
A Pilkington risk can arise where you already have a planning permission, say for a large mixed use development, and you later obtain permission (ie not reserved matters under the first permission) for part of the site. If this later permission (assuming implemented) means that developing out the first in full compliance with its conditions cannot physically be achieved (say because you propose a form of development which differs from the originally approved and conditioned masterplan) then you could lose the benefit of that first permission. This applies even though you may have implemented it and developed out one of the phases.
There are ways around it, but it is important to be on the alert for the risk. As well as developers, this can affect investors and funders.