https://www.nationaltrust.org.uk/features/escape-into-autumn One per household, up to 2 adults + 4 kids, valid until end of November. Some properties are excluded, so check the details beforehand.
Do it, folks Both Wentworth Castle Gardens and Nostell are worth a visit locally, especially with kids of primary school age. Brutally expensive for a one-off otherwise but fill your boots for a freebie! You can always use the (similarly eye watering) cafés to support them with a bit of your money whilst you're there. Annual membership prices not horrendous if you think you'll go to either or both fairly regularly and if you're ever going to Cornwall it's worth it just for free use of National Trust car parks!