Thanks for the responses, I'm pretty sure it is the springs that are too weak, either by design, quality or degradation.
So I think I will follow up Colin's mulberry suggestion.
Remember that ride height and suspension is provided by springs and the spring bounce is controlled by dampers (often called shock absorbers but in reality they are 'frequency' dampers). Some think fitting different dampers is a cure, that for sure will slow the oscillation frequency but not the full suspension travel which is the condition you report.
A good instructor at college when I was an apprentice summed it up well when he said 'think of the dampers as giving the springs time to do their work'.....never heard a better explanation ;-).
