My in laws have a Cavalier, he's been very healthy apart from a heart murmur. Recently he's started coughing, not often if he's distracted but quite often if he's not. The vets they are using have little idea, so have treated him for a range of things. They finally x-rayed his heart and it's enlarged. He's on heart medication now but is still coughing.

Any ideas?

Did the vet look to see if he had Larangeal Paralysis? It's more common in larger breeds, but you never know. My last dog was coughing on exercise and the vet thought it was LP and had her in to find out. She did have LP, but not enough to cause all the problems she was having. An X-ray showed the enlarged heart! The LP, although mild, seemed to cause the coughing mostly. What meds is he on

See a cardiologist. A heart cough is distinctive and unlike other coughs.Soft and deep, not raspy or harsh like kennel cough. It is coming from low in the trachea where the enlarged heart is iritating it rather than high in the respiratory track like L.paralysis.

You say he's on heart medication, but is he on diuretics as well? Fluid on the lungs is a common side effect of heart problems and you do often need water tablets for that.

Initially my vet didn't prescribe a diuretic until I mentioned it (who's the vet round here)
Noticeable improvement when she had that added to the mix.

I agree that a diuretic could help.  My bitch is fine with that.  As well as her Vetmedin and Fortekor.

My girl was on Fortekor initially, then Vetmedin for her last year. Good stuff and really helped and gave her another three years