The law (just like most things in modern life) is run like a business now so..
Magistrates courts are not geared toward motorists pleading not guilty(too
expensive given the potential small return/£200 or so fine)Requesting a hearing forcing a P.C. to attend will usually kill it DEAD/if not take photos,
get independant (an AA man will do)testimony that the Officer 'must have been' mistaken.
NEVER accuse a serving officer of LYING in what used to be called a Police
court(for obvious resons).Adhere to the above and you should be OK.
ALWAYS REMEMBER THE OLD LAGS RULE NEVER ADMIT TO ANYTHING.