Beagle is correct about the leading edge fillets. They were on the pre-production batch of Mk 3s, 5 off, IIRC. I flew them at Rissington in 60/61. All the production airframes that went to Syerston had straight LEs. I don't know why they took them off -something to do with the stall or the spin I think. The marked difference in handling was that you could pull the LERX version deep into the stall and it still kept turning. On the other kind if you pulled hard the stall was abrupt and you stopped pitching straight away.
Dick