the National Trust for Scotland is offering 6 months free membership when you buy 12 at the moment. Seems to apply to all membership categories.
Only available at NTS venues and NOT ONLINE. When I joined today the lady found an email from Head Office giving details of the deal! This for some reason is not advertised, but is a genuine offer.
So for a joint membership, direct debit cost £54, which is £1.50 per month each...
Will also give access to other UK National Trusts, but not the free parking outside Scotland.