Hi Robin! I use grey acrylic automotive primer - cheap and tough as nails. You'll probably find Halford's easy to find. Doesn't make much difference if you use acrylic- or cellulose-based except in the smell department.
Most of the proprietary sprays (Games Workshop for example) are much the same kind of thing, possibly the exact same kind, just relabelled and sold at inflated prices - last time I checked over double.
Best results are obtained by warming the can first in fairly warm water and don't skimp on the shaking. If the weather is cold and damp spray in your garage for best results. You can ignore the common spraying advice of starting and finishing the spray off the figure and shoot short bursts directly at the model if necessary, the primer goes into recesses better this way as well as being less wasteful. Don't worry about it obscuring detail, even on a Hornet figure.
Einion