I saw the picture before I read the rest of the text. I recognised it as a box moth, as last week my wife found one in our garden and immediately killed it. We have several box plants and had assumed the box moth had not made it up to Derbyshire so far.
It seems not, so we now have invested in box moth traps, which seem good at trapping carpet moths but no further box moths.
You can pluck out the caterpillars if you really want to be diligent...