We've had the flashing replaced, and seemed ok, but we've just got the
leak
back after this week's strong storm. Again, it doesnt leak when it's only
strong rain.

First check that your guttering is fine and doesn't overflow in heavy rain.
Given that the water appears to be going down the cavity and being forced
out by the door lintel, you have 3 choices.

1. Get the water out higher up by installing a cavity tray above the
flashing.
2. Stop the water getting in (by fixing/upgrading guttering, or coating the
bricks).
3. Use internal guttering from the vents to outside, having cleared the
plasterboard out of the way.

But you seem to have considered most of these solutions already...

However, you've really got to remove the plasterboard and see where the
water is coming from. It could be from the vents/cavity tray, but it could
also be from where the conservatory roof abuts the wall. The flashing steps
do appear to be insufficently angled. Rain entering at an angle could easily
get behind.

Christian.