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As a first port I would lift the flags. Look at the possibility of installing a French drain.
My husband grew up in the Heatons, said their 1930s semi that had a void under the hallway, behind front door, used to fill a bit with water in very bad weather. There was originally a stream on his street, in fact it’s still visible in spots.
So take heart, that house is still happily standing! I would look at filling the void as a very last resort after exhausting all other options. Other suggestions here are good, also try the SPAB for advice on old homes, they have a helpline.