So, took me a while to get all the info...

Indoor Circuit A is an MB12AHL ~12m piping length
Indoor Circuit B is an E12SQ ~ 6m piping length
ODU is an MU4M27

Been doing a bit more troubleshooting;

When heating on circuit B - it heats, but nothing like it should do.
If however I run up circuit A to heating... the heat output from B increases dramatically - A also heats.

If I run the fan on circuit A while B is heating - I get heat from this unit as well which I wouldn't have expected.

Im beginning to wonder if the control lines for A & B are simply the wrong way round in the ODU
Im not suspecting a leak - the heat output is there, its just not provided to B unless A is running also

Confused to say the least!!