No real tradition here regarding Sundays from what I've seen. We would do one when the kids were at home but not as often with just the 2 of us. Coincidentally had similar to CT yesterday, marinated loin of pork with Brussels sauted in bacon fat.
Bought a vacuum packed boneless NZ leg of lamb from Costco recently. Cost about $26 or $6 per pound. That's around $13/kg or 10 quid/kg.