It's your employers responsibility to renew it as they would have received the reminder. However if you drive it - you should make daily checks as to the roadworthy and legality of the vehicle you drive such as lights, tyres etc this includes tax. Get on to them immediately and tell them to sort the tax out today - its a trip to the Post Office at best. They could have done it online if you have the new MOT cert - there's no excuse for them to put you in this position. If you park the vehicle with no tax or get stopped and you have no tax - it's inconvenience but the ultimate responsibility lies with the employer.