Update - looked like the bicarbonate/ vinegar and later drain cleaner worked, but it hadn't. Had another look outside and lifted the drain hole cover - the outside drain could be seen (the one above ground must be the original one) so bought a cable tool from Screwfix which went right through to what sounds like under the shower trap and turned the wheel a number of times then brought it out and the whole tube was clean. Did that a few times but nothing. The water goes down but not quickly.
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