Couldn't be bothered to read everything.
But basically you are stealing someones work, charging £3 and you came on a forum to ask if its ok. Shame on you!!!!!!!!!!!!

I don't see how it is stealing. The OP has tried to contact the buyer, can't get hold of him, and the buyer's customers have been waiting for months to have their windows cleaned. The new guy hasn't contacted his customers, who would probably not be impressed by his tardiness and may tell him where to go as he is already proving to be unreliable.
If a newbie went canvassing and got them, would that be stealing?
No. If I were in this situation and was prepared to take the wqrk back, having exhausted all possibilities of making contact with the buyer, I would.
If the situation were reversed, I would accept this as my loss and my fault for not doing the work in a timely manner.
John