So, summer's arrived and I've been driving round with the windows down, noticed a hissing noise from both front wheels when I go over even the smallest of bumps, recently MOT'd so I'm sure it's not a problem but was wondering what's going on...

Could just be the sound of the gas pressure in the shock absorbers... 

If the sound is like "psssss" and especially clear when going over speed bumps - it is shock absorbers and it is normal.

Normal I'd say, mine does it and it is quote pronounced

mine is awful too, it is enough to spoil a pleasurable summer drive.

Wonder if its possible to fit normal silent struts ?

My last to Superbs have done this and I would consider it normal.  I've noticed it mainly on larger VAG cars though.  My previous Octavias didn't have the same noise.  It doesn't bother me though - at least I know the struts are active and doing their job!