My female neighbour likes to hold her naked breasts against me and cuddle but does not permit full sex. How do I get her to go further?

You don’t “get her to go further.” You find out if she wants to do something else and then you abide by her decision.

Since she’s indicated to you that she doesn’t permit “full sex” (whatever that means to you), then you already have your answer. It sounds like you don’t like it, but those are the breaks. Enjoy her half-naked cuddles for what they are and expect nothing more.

If you don’t have consent, then you don’t have it. Don’t think for one second that pressuring and pestering this woman for “more” is acceptable: it isn’t. Pretending you forgot and attempting to get what you want later on is still skeevy and a transgression. If you’d like background information regarding why she doesn’t want anything else, you can always ask. You may not get an answer, though.

If she changes her mind, she’ll let you know. If this situation isn’t acceptable to you, stop accepting her half-naked cuddles and look for someone else that wants the “full sex” you seem to desire.

P.S.: Please don’t think that marriage or any other kind of committed relationship is a gateway to the kind of sex you want. That’s the old-fashioned, “my-partner-is-my-property” rape culture that ethical individuals like myself rally against. It doesn’t even have to be marriage for it to still be rape culture. You’re still not entitled to anything even if you’re dating, you’ve had previous sexual engagement that’s been halted, you once hooked up, you enjoyed a friends-with-benefits arrangement the other person doesn’t want anymore, or what have you.