Not sure I understand Sky's technical support's answer. Have a look here for a lot of useful information on how to use Netstat in the Command Prompt.
For obvious reasons run your security software to make sure the PC is free from malware, so it's not being used as a Bot, which would use your bandwidth.
I would certainly not be happy with any Sky technician's reply if your PC is 'clean'.
Run their Speed tester (I'm sure they have one) and if still slow then complain again.
Test on another PC and I bet the results will also show a slow speed.
Hopefully Sky aren't trying to fob you off.... nah they wouldn't do that to their customers.