Finally got it out. Thanks for the penetrating oil tip, even though I did not get to it. Having loosened one leg a fraction (I could see the water and lubricant move and bubble ever so slightly), I kept at the other leg and got it close to the same point. No upwards movement, just a little play forward and back / side to side. Using the front of the ladder I kept pushing each side with my legs to try and get the ladder to twist in the holes and alternating this with using the trolley jack to get some back and forth motion. Eventually I heard the POP as the ladder broke free. Using the trolley jack I was able to lift it out.
Observations:-
Think the screwdriver clearing between the ladder leg and the in ground support helped get that first bit of motion into the legs
I would expect that the lubrication did help, but that the penetrating oil would have been a better bet as mentioned by kiss4afrog.
using the front of the ladder as a giant lever to give a horizontal twist to the legs in the in ground support is where I got the breakthrough.
Using the trolly jack helped finally lift it out.
The trolley jack on its own simply bends the ladder out of shape whilst putting pressure on the in ground mount. This does little to help lift the ladder out. At most is loosens things up. In the end a combination of the trolley jack and pushing the rear of the ladder towards the pool helped give the combination of lift and vertical alignment to get the leg free of the sides.
Took about a day and a half to get this done, so I suppose "don't give up" is a top tip.
Thanks for all your help and support. Kept me motivated and provided great advice.