Fill the schedule A form carefully with no gap in timeline.
and do compare all the data you previously filled in Schedule A form when you applied at first.
and provide info of sponsor like phone bill , driving licence , citizen ship card, uni, college or school enrollment, passport showing birthplace if Canada, any document showing the ties to Canada as a proof of residence in Canada
and provide all this and mention all of them in a cover letter, and send it in paper mail to London.
Background check is undergo when your file starts to be "in process "
Hope it helps, good luck
Regards,
Zam