Thank you Trigger, on an *extremely* close inspection this morning I discovered that my spindle was in fact 2 pieces not one. Once I’d freed the collar off and re assembled everything with it at the correct end of the spindle, everything goes back together as it should, with a space between disc and fork leg.
The scary thing is, somebody has been riding the bike like it was, for long enough for that collar to be stuck firmly in place.
The Haynes manual shows a typical assembly, I initially assumed mine wasn’t typical.