Merlin does NOT have heart failure!
SUCH a relief tonight that the news is he doesn't have heart failure. He had both an echocardiogram and ECG and both results are almost clear,

and my auntie thanks you all for you prayers and wishes for her special boy!
When my Auntie took Merlin in this morning, the vet said he didn't think (from stethescope examination and case history) that Merlin had heart failure and thankfully the examination confirmed this.
During the consultation we had in the evening, the vet went through the echocardiogram and explained the function and workings of the heart and related this to the images on screen. I found this quite fascinating once I knew Merlin was OK, as I had never seen an ultrasound explained so clearly! There is a very slight leakiness of the heart valves but it isn' significant enough to be problematic.
There are however two clouds on the horizon. One is that there is still no apparent reason for Merlin coughing up fluid now and again and the other is the lump, which the vet did mention could be a lymphoma. Both these situations are to be kept a very close eye on.
It is a great relief though to know he won't have to take heart medication and his heart shouldn't cause any problems for him.
He has been shaved but not as much as I was expecting and you can't even tell without lifting the coat.