Id 'like to use an old mobile (vodaphone) dongle to pick up a wifi signal - is this possible?

Very few "mobile dongles" will pick up wifi! So unless that is a dual purpose unit, cellular and wifi, you won't use that to pick up any wifi period.

As a statement of fact, all external antennas that WORK are designed using specific measurements for the frequencies being used. Just "getting a strainer" is not accurate since they come in different sizes and will not all "reflect" to the same point. You can't just throw aluminum foil around your usb device either. TUNING has to occur or you may actually decrease signal not improve it.

There are indeed wifi adapters with good external antennas which are designed to increase range, they work. There are indeed good designs for 2.4 and 5 gig external antennas but chose carefully. The ones that are good set specific sizes for everything used in the design!!!!

If you are in doubt about that, review the ARRL Antenna Design Book, it should be available at your library or order one. Like all other science specifics matter. On all microwave (which 2.4 and 5 gig wifi operates in) measurements are critical. Math matters!