I ended up desensitizing a scanner's RX circuit using a TV-preamp once.Oh - and add the possible DC power on the line (12 volts?) going into your scanner...
My suggestion would be if you have a balcony or even large window facing the city to buy one of those yellow light stands with adjustable telescopic stem.Remove the lights and bolt on some external yagi antennas and point them out the window in your chosen direction.If you have a balcony then you can place it there on a tempoary basis to please body corporate.It's portable so you can move it around & fold it up when not using.Might not look good in the loungeroom when you have visitors.
I use one of these when I go on radio meets & was using it on the day we all met at Sublime point.John.
But you have to check there is no foil sarking lining the roof below the tiles, else it'd be a waste of time.