PROBLEM SOLVED! :-D :-D :-D :-D :-D
I was going to follow Richard's suggestion of Molyslip (& may still do) but I thought I should remove from the equation, the doubt about the OEP220. Although this was the proper stuff & came from a sealed drum, I thought I should try something else.
The very first leakage did occur with EP90 & I have seen many Humbers filled with EP90 not OEP220 that leak through the breather, but I thought lets try with brand new stuff.
All I could get from the local garage was Comma gear oil EP80W-90. "EP Gear oil suitable for 80, 85 & 90 applications that require a GL-4 spec." So I put this in, after 5 miles the TF/box has only just hotter than the diff. Where as before it was similar temp after half the distance, but at 5 miles much hotter.
After 10 miles there was some leakage but not from the breather. This seemed to be from the drain plug. The aluminium washer has been squashed up quite a few times now & now it was hot I was able to tweak the plug fractionally tighter.
After 14 miles no leakage at all. Not scientific I know, but I could put the palm of my hand on the TF/box for a second or two pretty hot but after 12 miles previously it was untouchable.
So I don't know whether this particular oil is thinner than OEP220 or the EP90 I had used or whether it has some additional anti frothing properties.
Although interesting to know the reason, I shall just use this stuff in the TF/boxes of Humbers in future.
So a big thank you for everyone's input. I posted it here rather than the British section as it was interesting to hear if other types of vehicles suffered from this.
The downside of these experiments are the hazards of lying on the ground at laybyes where the tarmac has received special treatment from lorry drivers ;-)