Complex sentence: The Ecclefechan Tart gained national prominence in late 2007 when the supermarket Sainsbury's promoted them as an alternative to mince pies at Christmas, and the tarts proved to be very popular with over 50,000 packs sold in November alone.
Split into simpler sentences: The Ecclefechan Tart gained national prominence in late 2007 when the supermarket Sainsbury's promoted them as an alternative to mince pies at Christmas. The tarts proved to be very popular with over 50,000 packs sold in November alone.