Hi James,
I haven't tried this myself, but I've been told that elderberry and blackberry is a great combination. I don't have a recipe though.
When we were looking for Elderflower this year, we found loads in Stanmer park - especially on the way up to Stanmer Organics.
If you want glass demijohns you can usually expect to pay around £7 but you can get plastic ones from the Internet for a couple of quid. eBay also has collections of second hand ones. If you can find a collection for sale in Brighton then it's definitely worth it.
Hopes this helps.
Beth