Suspension overhaul Advice welcome

Good evening ladies and gents.

On friday I went out on my SV (its only about 8 months old) and when I came back i wanted rid of it. However I went out for three hours today and it redeemed itself

So i've decided to keep it but think it needs something doing with the suspension. I weigh 17 Stone and sometimes have a pillion on the bike (she is much lighter) and wondered what I can do with the suspension (budget is about £600) I currently have the pre load increased one notch front and back but find the bike a little flaky when i'm pushing it hard.

I intend to replace the D220's with avon Viper Sports or similar and would really like the suspension to be as good as the tyres!!

Also will my insurance company void my policy if I change springs and shocks etc?

Cheers in advance for the help..