YYou should apply for your visa atleast 28 ddays before it expires. Depending on the ttype of visa application you are making, the visa processing time would vary. Once you have made an application your visa auto gets extended under section 3c of the immigration rules. However, employers dont know this. So they will insist on the proper visa. You can ask them to make an enquiry on the employers checking service once you have received a case id number from the Home office. All the best!!!