So after my ACE dumped its oil I set about finding the problem, extremely easily as it happens, the banjo bolt was loose by at least a turn.
Pitched pump, refitted pipe, followed bleed procedure. All good, definitely plenty of return fluid.
But I still have a really loud squeal as if the belt is slipping but it’s a new belt and everything was cleaned down well prior to refitting, no obvious issues with any pulleys.
Could it be a noisy ace pump?

Quite possibly, I've had a pump shear off just behind the pulley.
Replaced with a used pump and all was fine after that.

Should of updated, the squeal was the manifold, bolts loose and bolts missing