Forgot to mention..
As Trebor said, low level (LF) Spits had the wingtips clipped to reduce drag, medium altitude (F) aircraft had the 'standard', and high altitude (HF) aircraft like the Mk.VI, some MK.VII and early VIII's had extended tips.
I didn't know that about them being shot down for Bf 109's, but I can believe it.... By the same token, I wonder if a few early BF109F's got away before the Allies cottoned onto the fact that the wingtips had been extended..?!