Hi Stokey, Whereas i understand some criminal charges are worthwhile contesting in court, i would not consider 'resist arrest' to be one of them, particularly in this case as there are four police officer witnesses who are stating you resisted being arrested for an offence of violence. Also, the offence of resisting arrest is very straight forward and easy to prove. (Case Law also makes the offence easy to prove).
For reasons above I would be surprised if an experienced Solicitor did not try and get the charge of resisting arrest discontinued on a 'plea bargin' for a guilty plea against the index offence of assault and if rejected by the Prosecution, advise you on your chances of being found not guilty and losing the 30 % reduction available in sentencing.
Anyway, sentencing for the offence of resist arrest is commonly the higher end of a Community Punishment Order and a fine but this will obviously be effected by the sentence for the assault that you have pleaded to (are you still awating sentencing for the Sec 39 assault).
You need good mitigation before you are sentenced and hopefully your solicitor has discussed this with you. It used to be common for Police officers to receive letters of apology from defendants going to trial which included resist / assault police charges who would be advised to do so by their defence solicitors so they could use this as mitigation in cour prior to sentencing !!!!.