I've grown a cutting taken from an unknown plant in my parent's garden. Any
ideas what it could be?

The parent shrub is about 4ft high, 6ft spread. Silver foliage on long
whippy stems. Leaves are toothed and in pairs about 1-2" long and about 3-4"
apart, with light blue flowers similar in appearance to lobelia growing from
the leaf joint. I can't find it in my plant encyclopaedia. It was planted
when the house was built so is probably a pretty standard landscaping plant.
It was next to phlomis ( Jerusalem Sage ), and a Photinia ( Red Robin ) if
the context helps.