//Also if the offence is either way, at what stage does the defendant know if it can remain in magistrates?//
If a defendant enters a "Not Guilty" plea (or declines to enter a plea) to an "Either Way" offence, the Magistrates undertake an "allocation" exercise (formerly known as "Mode of Trial"). They hear an outline of the allegation from the prosecution and then hear from the defendant regarding any lines of defence. Using their sentencing guidelines they then decide whether the trial is suitable to be heard in the Magistrates' Court of the Crown Court. This decision is essentially based on whether they consider their sentencing powers would be sufficient in the event the defendant is convicted. If they decide to retain jurisdiction the defendant then has the right to a trial by judge and jury in the Crown Court. If they decide it is too serious for them to deal with the defendant is sent to the Crown Court for trial. The defendant has a right to a Crown Court trial if the Magistrates decide they can retain jurisdiction but he has no right to a Magistrates' Court trial if they decide it must go to the Crown Court.
A defendant pleading guilty, or being found guilty following a trial in the Magistrates' Court can be sent to the Crown Court for sentencing if the Magistrates believe their powers are insufficient.
Normally a solicitor would instruct a barrister to represent his client in a Crown Court. However, since about 1971 (I think) solicitors have had the right to represent their clients in the Crown Court if he is appearing there either having been sent for sentencing from the Magistrates' Court or is conducting an appeal against a verdict and/or sentence handed down in the Magistrates' Court.