Thank you all for your answers! I now have the accurate ref. Scrolling down would have saved me exploratory time had I been in a hurry though as the results are listed neatly at the foot of the page.
You can also use the Ordnance Survey's own site at www.getamap.co.uk Log in as aguest, enter your postcode, move the map around so that it's actually centred over your house and it tells you the grid reference of that point.
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