Got straight back on it today. First off, make a second plate to double up and spread the pulling force. It will now wrap around the column below the wheel and bolt the two halfs together.
Put it in place and on with the bearing puller...
Started to tighten up and watched the steel begin to bend under the strain. My heart sank as this was really beginning to get ridiculous now. However, I persevered and just as the puller was on the verge of coming off the bending plates, donk! Movement at last. I quickly reversed the plates, refitted the puller, and began to crank. Slowly the wheel finally eased up off the shaft.
Well, what a relief. Can't believe such a stupid problem could cause so much grief. Loads of time and effort spent trying to undo something that should have come apart far easier.
As I suspected, the splined shaft and bearing surface is corroded...
I was also correct in doubting the penetrating oil was getting in; it clearly hasn't. Tip for anybody re-assembling or putting nuts on bolts; put some bloomin grease on!