@Vezeley, I suspect only sheer laziness. :-)
@Kwaaked thanks for the tutorial. It answered my question that I do need something else (baking paper, ironing cloth) to protect iron from the fusible.
@Ploshkin, of course Freezer Paper, something I never find in normal life in the UK.
@ BrendaP, not got any so won't use it!
I was wondering about using the generic interfacing because it wasn't as good in garments as the Vilene (a certain cardboardiness, though I suppose that could be good in bags....)
I will let you know how I get on with it.