unknown caterpillar / larvae

unknown caterpillar / larvae

Observation - unknown caterpillar / larvae - UK and Ireland. Description: I found a number of these caterpillars eating holes in the leaves of garden mint. My caterpillar book mentions a mint-eating caterpillar but it looks nothing like this. When I first found them (could not get a photo) the tail was hairy, but now it looks

I found a number of these caterpillars eating holes in the leaves of garden mint. My caterpillar book mentions a mint-eating caterpillar but it looks nothing like this. When I first found them (could not get a photo) the tail was "hairy", but now it looks like congealed "goo". The main body is about 5mm long (excluding the tail, which it waves about if you get too close).