After much discussion as to what form debating extending Sunday Trading hours will take - it now looks likely that it will be discussed in the House of Commons next Tuesday (17th November). The SNP have now joined the opposition to the Bill, so there is a very real chance of defeating the Government on this issue.
We at Christians on the Left would encourage you to contact your MP if you haven't already done so. You can find a handy tool for doing so here: http://www.keepsundayspecial.org.uk/get-involved/communities
Whilst you are waiting for the Bill to be debated, here are some links which explain the situation as it stands.