To connect your Laptop wirelessly to your in-laws' broadband they would need to have a *Wireless Router* installed.
>>> Am I right in thinking a USB dongle into the
>>> PC could fire out a wireless signal for my
>>> Laptop to connect to or am I simply being silly???
Yes, you are right. Provided the PC is running and connected to the internet, you could run an "Ad Hoc" wireless network between the two computers, enable "Internet Connection Sharing" on the PC's actual internet connection (modem connection) and have simultaneous internet access in your Laptop. I have detailed step-by-step instructions for setting this up at click here although these are actually for XP rather than Vista. See how you get on and post back if you need more help for Vista.