Over the last few weeks I've been occasionally squirting a bit of the specialised WD40 on to the top seat tube/ well stuck seatpost, and once or twice through the bottle cage boss holes with the bike upside down. Not sure if this has done much, but maybe...
This pm, having ridden at this saddle height for a few years, I finally thought I wanted to try the seat a tad higher.... So, with a rush of blood to the head, I took the B17 off, and using a lump hammer with and without a block of hard wood on to the top of the seatpost, I managed to whack the seatpost into the seat tube a couple of inches - thinking I may loosen the 'bonds'. Unfortunately I couldn't whack the seatpost up. I then stared into the 'it wasn't that broken, so why did you try and fix it' abyss... I coated the seatpost with WD40, took a large pair of mole grips and clamped them on to the angled top of the seatpost, and then managed to turn the seat post in the seat tube a little. So I added squirts of lithium grease, and kept on turning, until with some force I was able to do full rotations of the seapost (along with some horrendous metallic squeaking). I kept on doing this for a while and then started to add upward pressure, and found using a measuring tape, that I was actually lifting the seatpost bit by bit. A while later it popped out! New (original) seatpost now installed (well greased), and saddle higher to try - leg locked out with heel flat on the pedal (unlike before), with hip movement OK. Testing tomorrow.
Not suggesting this a method to follow, but worked for me.