That's a video by Toby with help and advice on Pork Shoulder/neck/butt. Granted it deals with a "proper" whole joint but it's well worth watching especially with regard to your question about the fat cap.
As to the string, the larger the joint you've bought the more likely it is to be in one piece, smaller joints may well have scraps added as fillers to bring up the weight, although I've not noticed that on Asda's joints. If you have suitable string and feel confident to re-tie it if needed then have at it. You don't need to re-tie it perfectly for presentation should you need to tie it.
Good luck, happy eating and have fun, and a few tipples


