You'll need 4 to 5 ginger nut biscuits per person you're intending to feed. Like I said, I used always to use sherry for dunking, but, as I write this, I'm thinking about using pineapple juice and maybe adding a little bit of crushed fruit in between the biscuits and cream as well. The Couldrons & Cupcakes lady adds a tablespoon of honey to a 300 ml pot of cream and maybe that helps the cream to whip up nice and thick, I haven't ever tried doing this myself. Refrain from dunking the biscuits until the cream is already stiff and even then be careful to only dunk them fairly briefly or they will turn into so much mush, you won't have anything left that you can stick together. It's important to dunk them for long enough that you aren't left with any hard bits in the middle though. Once it's finished, you can tell your friends all about how difficult it was to get the texture exactly right; I can't guarantee they'll believe you though.
Two other links you might want to check before having a go at making this one are at Student Recipes and Taste Magazine. A bit of crumbled flake over the top looks pretty, or I'm thinking toasted coconut flakes might add a certain something to a log built out of biscuits dunked in pineapple. Be as creative as you like, and do please let me know how it turns out.