As it turns out I had some bits of 6" x 2" - can't remember where I aquired them, but far too good to leave in the skip...
Replaced the cracked side only -
cracked side removed:
Using old beam as template for holes:
New section in place...
...noticably longer than old piece, giving more strength at end grain hopefully:
1" ply load-spreading top plate refitted (tight push fit):
Better left for that bit of extra strength than sawn flush?
Hopefully this will last until I can make myself a better one from steel... just 290 litres to go ;)