That is my main issue of course but my inabaility to set sag for my weight etc has to be springs as with the bike static surely oil level should not play a part IMHO?
That depends. The air doesn't have anywhere to go, so it has to compress as the suspension compresses. As one can see from operating a hand-pump with the outlet blocked, a relatively big volume of air, is very easy to compress initially, but gets progressively harder (exponentially). A smaller volume of air will start out a bit stiffer and get much stiffer, much faster. So it all depends on how overfilled it is. If it were full, the suspension would have to compress the oil to sag, which it can't, so you'd have essentially a strut. If it's not entirely full, it might be, that there's little enough air, that it can support yours and the bike's weight, without having to compress enough, to get proper sag.
Is it likely? Well, I suppose it's not unlikely. I'm not sure of a figure for the proper air volume in there, but it's not much and, in any case, all it takes, is a botched up fork service job. On the other hand, a linear spring would have to be pretty stiff to support you at a sag of 25mm with no external preload, unless either the internal spacers, or the springs are significantly longer, than they should be. I don't think you'd be able to find one. Can you compress it much farther than the sag point, by pushing down on it, or does it more or less lock up soon after 25mm of sag?