Hi DFC
The way I do a restaurant style de-boned leg of Lamb, which will work just as well with shoulder.
Give the meat a good stabbing all over, at least half an inch deep stabs.
Two Marination methods (options)1st marination =
Do a Rub with a mix of Veg Oil, plenty of G&G, 2 tsp Mixed Powder, 1tsp Chilli Powder
0.5 tsp Black Pepper, 1 tsp Salt and a handful of finely chopped coriander.
Rub it all over inside and out and pushing it into the cuts. Cover it and let it sit for at least an hour.
2nd marination =
Mix a 200g carton of plain yoghurt, plenty of lemon juice (Big Fresh Lemon) G&G and spice powders
= 1tsp Madras Curry Powder. 1tsp Coriander Powder. 1 tsp Cumin Powder. 1tsp Garam Masala.
A handful each of fresh Coriander and Mint Leaf if you've got any, blended to a paste with 4/5 Green Finger Chillis
and a 2tbsp of Mustard Oil. Add this wet mix to the marinade.
Same again, rub it all over inside and out and pushing it into the cuts. Cover it and let it sit for at least an hour, before you cook.
Right, pull off a few feet of Ali Foil and wrap the meat as tight as possible creating a tight parcel of a few layers so its completely sealed.
Tie it with a bit of string to keep it tight.
Now its ready for the oven or slow cooker, timings vary, but you probably know the score,
This recipe is from Bangladeshi BIR Kitchens, where the simple ingredients are all at hand.The first marination version is the normal method and is great.
Going onto the second marination version I was shown in a top class Bengali Restaurant in Newcastle and is even better.
Let me know if you try this, but don't tell anybody because its from my new EBook.

cheers Chewy
P.S. Marinating for days is a bit of a nonesence, a couple of hours is all you need.