The Agapetus is a micro Caddis fly - it's only 5mm long as a fully grown adult!
These flies are mainly found in East and North Yorkshire (and especially the rivers around Driffield)- I've not heard of them found in any other rivers in the UK.
This fly is available in either size 20 or 24.
Choose your size and then click "Add to Cart", you can adjust the quantities in your cart.