I noticed this on my window earlier today. I’m surprised as it’s not in a very sheltered spot. I’ve had a go at identifying it and think it might be a crysalis of a small or large white. The life cycle or life history of butterflies and moths goes like this (common term first, then scientific term is in brackets): egg (ovum), caterpillar (larva), chrysalis/pupa, then finally adult (imago) butterfly or moth. I’ve asked some friends of the OP Wildlife Project far far more knowledgeable than me to have a look too.
If you come across any during the cold months, you can have a go at identifying using the guide from UK butterflies:
and for more information on the stages of butterflies and moths see: