Heya, Nat,
I'm afraid this is a pram so hard to get your hands on and so expensive I don't have experience with putting it back) together, I am sorry. I did find the Silver Cross instruction manual for your to check and perhaps find some answers, or I can recommend emailing or calling (0345 872 6900, Mon to Fri between 9am & 5:30pm) the Silver Cross customer service who should help you even better than I can.
If, by any chance, there's a part missing on your Heritage model, there are two stores in the UK - contact is here, who keep those and help with repairs, so they should probably even help you with putting it together, too. Don't hesitate to call them.