I am not an expert, and it is while ago since I was a union workplace rep, so please seek legal advice or if you are a member of a union, their free legal advice line (I always paid my union subs just for that!)

Until sentence you are under the law innocent and not convicted. If it says you have to disclose convictions in your contract, I would have advised a fellow employee in your situation to disclose. It is possible to argue, if as you say, that driving in not an essential job qualification, you have fully owned up to your culpability and throw yourself at the mercy of HR. However, I think that failure to disclose would amount to gross misconduct, which is an "escorted of the premises" breach of contract if they should find out. 

Each individual company and case is different, so you will find getting hard and fast advice tricky.

I am pending sentence for a drink and drive offense currently , was low limit so likely 12 month ban and sentence. I never read my initial contract and once i was charged read and it says during my employement its my duty to disclose offences . I am now confused whethere i need to or not for this offence and also when ? should i wait for court hearing ? or wait after sentence to see if it reflects on my records ? i am in a maangerial position and driving is not essential to my job and it wasnt a company car . I believe honesty is the best policy ( i have clean criminal record and no black mark within the company) genuinely i dont know , if i dont disclose its likely they wont find out as i am moving fully out of UK to a different role in another country within same organization (or) is it best to declare and face th consequences , how likely are they to fire me for this type of offence?