Just had to do one of these on supply chain security in the defence business so, while I may not be able to give you any specifics I'd suggest the following.
A SWOT analysis follows out of internal and external analysis of Sainsburys and the wider arena (how wide is entirely up to you, more in a moment). If you do a SWOT cold then a) you bring your preconceptions to it and b) chances are you haven't really looked in depth at some very interesting possibilities.
Space doesn't allow great detail but a web search should help. Start with internal analysis. This looks solely st Sainsburys. Start with a Cultural Web Analysis - this looks at symbols, organisation, stories ,power structures etc. to get the paradigm of Sainsburys. I did an As Is and a To Be (what my ideal paradigm would be) comparison.
Then do a Stakeholder Analysis (which might include external stakeholders), This can be written or plotted (you're after low/high power and low/high interest comparison here. Then a Value Chan analysis. Written or diagrammatic.
External Analysis includes a PEST (PEST(E) or PESTLE) analysis - really let your imagination run right here. You could even bring some of the Delphi Forecasting technique to this. Political, Economic, Social, Technological considerations are made - as wacky as you like. What will change in teh next 10, 20, 50, 1000 years.
Boston Grid (or BCG Grid) is next (don't have to do in this order) measures Growth Rate and Market Share.
Last External is Porters Five Forces - looks at alternatives, competitors (that'll be Tesco then!), suppliers, customers and identifies where they hold power.
From this you have done your Strategic Analysis which is brought together in the SWOT.
Next step, if you like, is something like Ansoff's Matrix which identifies/refines strategic choice - what you could do. From there it's strategic implementation time!
Hope it hops!
Do a web search - Wikipedia might be a good start