See pics here: http://insectchrispsl.wordpress.com/2014/06/20/leaf-miner-on-nasturtium/
No help from http://www.leafmines.co.uk/ or http://www.ukflymines.co.uk/Keys/NASTURTIUM.php the links in the latter don't match. There are shots of similar mines available online but no determination of species.
Any ideas?
Guess you'll have to try and rear it Chris! If it does turn out to be Agromyzidae, Dave Gibbs is an expert on the family.
He's also an expert on many other families (and orders and even kingdoms!) it would seem :)
Well it helps to look at the right plant:-) you have confused the Genus Nasturtium of Water Cress, which you searched on UK leafminers, with Tropaeolum, the vernacular name Nasturtium, the plant in your photos. If you search under Tropaeolum you will see several Drosopila and Ephydrids listed, yours are probably one of latter.
D'oh! Thanks David.