Address failing mypy tests

[ukmo-ccbunney]

📰 Custom Issue

Following a QA refresh of this repo, the following MyPy errors are being generated (and are currently being ignored in pyproject.toml):

Tasks

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1. attr-defined See check-that-attribute-exists-attr-defined
2. misc See miscellaneous-checks-misc
3. no-untyped-call See check-that-no-untyped-functions-are-called-no-untyped-call
4. no-untyped-def See check-that-every-function-has-an-annotation-no-untyped-def
5. unreachable See check-that-statement-or-expression-is-unreachable

These should ideally all be addressed if possible.