Have a look at the dampers (shock absorbers) do they look as if they are leaking fluid?
How many miles have you done?

The top mounts under the bonet have a look at them while someone else turns the wheel they shudnt move

I have done 15.500 miles and it's only just started to happen. It feels like the agp standard before I had the ss kit put on?

I would imagine that one or more shock absorber has failed/is failing.  They tend to get get weaker with age.

Is the car sitting level? If not then check the springs to see if one has broken.

Can't see it being the top mounts to be honest, they do fail frequently but normally just cause noise and maybe a slight bit of movement, not enough to make you think your suspension has gone soft.

Other thing is tyres, some makes have very soft sidewalls but if you haven't recently changed them then it rules that out.

As for trying to pinpoint where the problem is, does it feel softer when cornering one way more than another? When braking does one side of the front dip down more than the other?  Another good test for the rear suspension is to drive at a few miles per hour and slowly apply the handbrake firmly but not enough to lock the rear wheels, does one side of rear of the car dip down more then the other?

Good luck   

I will try everything that has been suggested. And many thanks for all your replies :-) watch this space.

I went to my dealer today and he road tested the car and admitted there was a problem and he adjusted the balancing of the front suspension electronically. And told me I had too much psi in the tyres. I had 37 psi in the front and 33 psi in the back. I got these pressures off this site, but was told this is incorrect. The correct pressures are 33 in the front and 30 in the rear. The technician has told me there is no leaks or damage to the shocks? But the ride still feels the same? I really don't know what to do? Is it the springs? The technician did road test the car and said it was fine,but I know from driving it home that it doesn't feel the same,still spongy over speed ramps ect?

Lloyd I wish I knew, but the technician assured me that 33 for front and 30 for rears! He told me that is what abarth recommend and that running the tyres on the pressures I was running,would do more harm than good!! I did ask the question on here and I assumed that was correct? I'm still thinking to get the springs changed as I'm not happy with the ride still