I stripped down the old engine this morning.
Oil has been leaking from the oil breather and rocker cover.
Started by removing the cam belt, you can there was oil on the tensioner and cam belt cover.
Removed the oil breather and rocker cover
Cylinder head removed
Bores look ok and you can still see honing marks.
Then flipped the engine over to remove the sump and discovered this. Oil pickup was blocked with what appeared to be gasket sealant.
Cleaned the pistons and upon inspection two were cracked. One with a large piece missing, not sure were that went or which cylinder as there is no obvious damage to the bore!
The other two pistons were in good condition and all the rods were all straight (surprisingly).
I guess the oil pickup blockage has been starving the engine of oil causing the piston damage and the excessive oil consumption.
I'm definitely glad I've got the new engine now as it looks like I was lucky the engine didn't blow up!