There has been a church on this site for over a thousand years. It is quite remarkable that the nave and aisles of the original 8th century building remained largely intact until 1841, and had they survived to this day, Lady St Mary Church would be the largest surviving Saxon building in Britain. However, it was not to be, in 1841 the roof had fallen into dangerous disrepair and much of the church was affected by damp and so the decision was taken to demolish the main body of the church and replace it with a more substantial, lighter and capacious nave and side aisles. By and large, this is what we have today.

https://visitpurbeckdorset.co.uk/activity/priory-church-of-lady-st-mary/