Keep any receipts for the dongle.
Once you have finally been connected, you should then approach Sky and ask how you can go about claiming a refund for the cost of any top ups.
Unfortunately I can't guarantee how much you are likely to get back, so be careful.
Unfortunately it was a BTO Engineer who has let you down. I have heard similar stories occasionally posted on the web where a BTO engineer has not attempted to knock on the door, I assume he/she has gone elsewhere for another job and claimed that access to the customers property was not granted.
Perhaps he/she did knock and you didn't hear? More likely they got the wrong home.
A good method around this for anyone reading this thread is to make sure that someone has passed on a mobile phone number as a method of contact.