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Hey. On my last service was told my car had a reasonable amount of sludge build up(blackdeath) in the engine. The mechanic added oil additive and cleaner and said i should use it every service. Just wondering if you agree? Also I was thinking a engine flush may help? But I've read they can do more harm than good? Advice on this would be appreciated. Thanks

Unfortunately, there's no quick fix for this problem.
Once an engine has a build-up of sludge, the oil galleries, oil pump pick-up and other components become clogged, but dislodging the build-up with a flush can cause further problems. With hard particles flowing through the oil, bearings can be damaged and oilways blocked.
The other problem caused by sludge, is oil seals and rubber components become very brittle as they haven't been properly lubricated, so the seals fail to operate correctly and oil leaks develop.
One possible (partial) solution we have heard of is to use a high detergent oil (such as a diesel oil) and change it every couple of thousand kms. This has the effect of slowly breaking down the sludge over time - however, there are no guarantees.
As you can see, the only real fix is to dismantle and rebuild the engine (or replace it with a clean engine).