Are you saying that it's now working ok?

Yes.

Its not a very good idea sending "spent" CH water round the HW circuit,
because it will already have cooled a bit, so HW recovery will be slow. At
the point where the return flow from the CH rejoins (what is supposed to
be)
the return flow from the HW, it might be an idea to use a sweep tee rather
than an ordinary one. If fitted with the sweep in the right direction it
would be far harder for the water to go the wrong way.

How near the boiler is the point where the circuits split? Does your
boiler
have 4 connection points, two of which are currently blanked off? If so,
and
if the split is fairly near the boiler, you could consider having two
totally independent circuits all the way back to the boiler with no shared
pipework. That should definitely fix the problem.

No boiler has 2 connections.

As it is working now (i.e there is hot water) - I will leave it as is for a
week or too - and then replumb the system.