Hey Mikey_S, many thanks for that, very useful and I have now removed the gaiter.
The perished part of the gaiter had actually been nibbled away by something - a mouse? - as had the ring of insulation material which sits under it. As had a small bit of the outer side of the rubber boot which sits under the insulation. Nothing dead, though, within the void. How it got into the car God alone knows, maybe through the heating vents?