Simon,
Humbrol Primer isn't in my paint box - I've never used it - I often use automotive acrylic primer.....sounds scary, but I've found that it works really very well. Etches the plastic, provides excellent "tooth" for the color coats, dried REALLY quickly, and no model paints can harm it, so you can spray ANYTHING over it.
For simple structures and builds, it's great - but it can slightly attack areas that have a lot of filler, making the filler show up. In that case, I useModel Master primers like light greys and flat white. They work really well and I've never had any problems with them.