can the physio be ordered to leave the sideline, as he is the only person trained to treat injuries and administer first aid i personally think this is wrong

I'm afraid so.
Look at it this way - if he knows he/she is the only one who can administer treatmant then they should not look to get involved in anything that may incur a 'sending off'

On the other hand if the ref sent the physio to the pavillion area and someone was badly hurt they might allow the physio over - do the deed but to go back to the pavillion afterwards (but that's at the ref's discretion)

What is the alternative - allow the "physio" to castigate and berate the poor referee knowing he is "untouchable" - I don't think so. If he is occupying a post within a team he should be treated like any other player and official with the same consequences. Should an injury occur I'm sure the opponent's "physio" would help our. Committee members of all clubs should be supportive of the referee and show repect - or am I being too simplistic?

Shootie-in. Abusive physio. Send off. Deal with as official. He's responsbile for consequences to injured player. Not the referee. He'd just be dealing with him like any other official. johnspeaker quite right.