Thanks guys, I've sealed the damp putty with LS-X, will pack the collar with new putty and wrap with denso tape. If it still leaks after that I'm going to cry!
Help! While retiling my bathroom recently, I nudged the pan and subsequently managed to disturb the seal between the pan and the lead(?) soil pipe which then started weeping a drop every flush. The sealing material (caulking?) is not too clever, ie very crumbly and damp especially at the bottom...