UPDATE: NT17004M0 gives you a £15 National Trust gift card, to spend at National Trust locations (some small-print restrictions will apply as usual). Think this is a much better option in my opinion.
£5 M&S E-Gift card and a pair of National Trust binoculars when signing up to a direct debit plan either monthly or annually
Direct Debit payment
Been looking for an offer on NT, only one i can find, better than nothing i figure. Quidco offer £5 cashback but reviewers seem to indicated these are being declined.
I know it'd be cheaper to buy a membership from a different country with a reciprocal arrangement but that's not for me.