Waitrose do a passable Boston Butt joint for about four quid. It's covered in a dry rub and you cook it in the oven for a few hours.
I then shred it, drench it in BBQ sauce and serve on a bun with coleslaw.
I like the Waitrose joint too, relatively good value for money.
Just done double of the Morrison's £1.50/Kg joints in the slow cooker for tea tonight. Dry rubbed it with spices last night, seared it in a pan a bit this morning and then whacked it in the slow cooker with a couple of cans of coke for 10 hours. Lid off for the last hour to let the liquid reduce, then shredded it and mixed with BBQ sauce. Ate it in texmex style burritos with refried beans and the rest.
Ended up with 3/4 of it left over.