It is common, as illustrated by some of the above replies, that the OEM cast iron exhaust elbow on VP “D”-series engines disintegrates when having heavy deposits removed. They seem to last between five and ten years. I know that the next time I touch mine I will need to replace it, as last time I cleaned it I noticed that the spigot where the cooling water tube attaches was corroding at its end and was only just long enough to be serviceable.
In the event that your exhaust elbow does not survive being cleaned, here are some alternatives that will cost you less than £400.00:
https://www.parts4engines.com/volvo-penta-d1-30-stainless-steel-exhaust-outlet-kit/
https://www.ebay.com/itm/234421291374
https://fybmarine.shop/volvo-penta-d1-30-series-exhaust-elbow-part-number-861906