I have answered on the Thalassa forum but have copied the information here in case anyone else is interested.
Mercadona is not open on Sundays at the moment - 9am - 9.30pm Monday to Saturday are normal hours. (Public Holiday hours are different)See their website -
www.mercadona.es
Lidl is also not open on Sundays. Normal hours 9.15am - 9.15pm
Leroy Merlin in Cartagena is open Monday to Saturday 10am - 10 pm - Tel No. 968 149 800
Carrefour in Cartagena is open Monday to Saturday 9am - 10pm
Toys R Us in Cartagena are open 10am - 9.30pm Monday to Saturday but also appear to open 10am - 10pm on Sundays. Telephone no. 968 197 034