Where do I stand with the I must not be in the house when they come ? Its obviously my house and the PC they need to use is mine, and I wont be allowing them to use that If I am not supervising them.....
They're well within their right to only speak to/deal with the account holder. I don't think the letter so much dictates you cannot be there, but you aren't supposed to interact with them either. Just tell your wife the bits she needs to know and let her get on with it, I'm sure she isn't stupid.
At the end of the day, stupid people or not, they're coming to a strange house with people they don't know to attempt to fix a problem, if you're acting in any way angry or agitated - I know I'd be pretty on edge if a customer was watching my every move and after seemingly a few minutes of restarting equipment asking me to leave...