Could be seat pin as above. Try a very thin wipe of good thick lithium grease on the rails as well, might stick around longer than oil. Also thoroughly clean the slots in the clamp that hold the rails. Other than this you might as well take the seat pin out, take the saddle off, and undo everything. Then clean and reassemble with grease or anti seize. The underside of saddles and the nice little nooks and crannies that are there are an absolute mud magnet.
I’ve also rigged up a stereotypical ‘old inner tube’ mudguard. One end ziptied around the seat pin low down then opens out and ziptied to the saddle rails at the rear. Doesn’t look mega attractive, but I runs reverb and it’s just too damn expensive a bit of kit to be wilfully left to the mercy of the British winter.
Good luck.