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Phil L

wrote:
Hello.

I have a 1940s house, so even though it has cavity walls (I'm still
learning about all this) can cement rendering cause dampness on
internal walls?

No, damp is caused by water getting in, either by tracking across the cavity
if it's been breached, or by a failed DPC

My kitchen and extension's external walls are rendering. One part is
"hollow" though it was fixed and the dampness is more noticeable
after rain. Also both rooms have a concrete floor.

If it's 'hollow', it's not been fixed, otherwise it wouldn't be hollow.

Where does the external render finish at the bottom? - it needs to be above
the DPC not below it.

You may need to hack off the hollow render and do it again, also you need to
check the internal plaster that it's not all the way down to the concrete.

More information required for a proper diagnosis