I find the NT really helpful Tegan. Their helpline seems to be manned by Liverpuddlians who are very friendly and extremely helpful.
Agree you do have to have 5 years continuous membership to get a discount. If you add grandchildren to your membership it costs £5 regardless how many you add. I renewed my membership in September and asked about adding DGD who would be 5 the following month. I had to apply again on her birthday and they waived the £5 until I renew next year.
I've got another 'keen'... Ouch!