Hi Phillip .Thanks for your information. I can now confirm that after installing a new elbow ,all is now good.
I did start to clean the old one up , but with the knowledge of trying to clean cast iron heat exchangers once they have scaled or blocked . They always leave a rough surface ,which tends to accelerate the problem when re installed . I managed to get a replacement on eBay ( metal finish ) for £195 inc gasket and delivery . A tin of Volvo green engine spray paint for £20 .
I run the engine at full revs for 15 mins under load . And No smoke with a higher water flow rate. I’ll be making this a part of my service schedule n future . Thank you all for your input .