Dozy - a bit more information for you.
I spoke to a DVSA Enforcement officer friend and he gave the following information.
His advice is to attend and plead guilty as you definitely knew what you were doing and have probably already admitted so under caution to DVSA anyway.
If you do not attend and do not enter a plea expect around £440.00 fine plus costs of around £200.00 plus victim surcharge of £44.00 (10% of fine). This is the ‘average’ for a level 4 offence.
If you plead guilty but do not attend expect around 30% less on the fine and therefore slightly lower victim surcharge.
He also said that by attending and pleading guilty but putting your full story across including mitigating circumstances and your desperation to do the right thing etc - it is not unheard of for the fine to actually be less than the maximum roadside fixed penalty of £300. It all depends on the individual magistrate and their like/dislike of HGVs.