In general VFR flight in a control zone requires 5k vis, but your licence requires 10K for special VFR, so you might think you would never get to fly in controlled airspace in less than VFR minima.
However the ANO goes on to make an exception for a route or zone which is "notified". Fortunatly most class D zones are so notified and allow a PPL to fly special VFR down to 3k vis. So in such a zone you could ask for a VFR clearance down to 5k vis and special VFR between 5k and 3k.
The above aplies to day time. If you have a night qualification, then all night flights in control zones are either IFR or Special VFR however good the vis.
If you have an instrument rating or IMC rating you can accept a special VFR clearance down to as little as 1800m vis.
As a basic PPL holder it is worth remembering that ATC dont ask what your qualifications are when issuing clearances. If the vis were say 2k they might well offer you a special VFR clearance, but you can't accept it. Below 1800m they will not issue special VFR to anybody.
For the "notified" zones check UKAIP ENR 1.4 - 2.4.1.1 note 4
Last edited by Rivet gun; 24th March 2006 at 18:41.