Confidence: It might be this.
Notes: Keyed out using the key at http://www.bladmineerders.nl/plantenf/pfsalicaceae/populus.htm. The frass line seems green, the corridor mine long and sinuous not appreciably narrowing. It ends in a small fold in the leaf.
Not a species I have seen before, and quite a recent arrival the first Middlesex record being in 2010.