I hope next year, you will try to identify the hedgehogs by natural means to save having to mark them. It should be possible to identify 20 individuals quite easily, especially as you are seeing them in daylight and they will look better on your films if they look completely natural. If you are going to mark them, I quote from a message on Twitter from the BHPS (British Hedgehog Preservation Society):
“If people must mark ‘their’ hedgehog visitors, all it needs is tiny mark with water based product away from face……”
I would add to that, on the spines and away from the skirt. Water based product is thought to smell less than nailpolish.
I am not on Twitter, but you can access the BHPS Twitter on the Home page of Hedgehog Street. That ‘tweet’ was from 23rd March 2017 in response to some poor unfortunate hedgehogs which had been excessively marked.