If you were employed as a cover supervisor then you shouldn't be doing any teaching you were there to supervise. I would stand firm on this and tell the Agency if they want to send a teacher then they need to pay the rate for a teacher. As for being critcised for reading the text book required for the lesson it sounds to me that they were being very desparate to find something to complain about. This particluar agency comes across as a particularly stupid bunch of "numpties" and I would tell them politely to "sling their hook". There are some good agencies out there I have found "Teaching Personnel" to be always good for me, although I accept the service you get might depend on your particular region and consultant. I know that you cannot name bad agencies, which is a pity because Supply Teachers/CSs would like to know which ones are a waste of space.