Clearly this is not a wandering individual and there are I think two possible explanations for this number at Clough Wood. Either they bred here last year and these are the off-spring or they have been deliberately released in the area.
There is no way of knowing which explanation is correct but it will be interesting to see what happens in the remaining flight period this year and if the butterfly occurs again next year.
In common with most people with an interest in wildlife I have mixed views on re-introductions. Some such as the White-tailed Eagles in Scotland and Red Kites have been highly successful in reintroducing birds to areas where they formerly bred but I am against the reintroduction of none native species. Extending the breeding range of our native butterflies is somewhere in-between. It would be great to have the Marbled White as a local breeding butterfly but a deliberate reintroduction somehow diminishes the record. I dare say that if they are still there in a few years time I won't worry too much about how they arrived!