I personally think that commercial grey screens are rather expensive. IF you want to see how a grey screen will improve blacks/contrast then why dont you pop down to B&Q and pickup a piece of MDF which is decent size but will fit in your car. Then pick up some ICESTORM 5 paint (go to the paint mixing desk at B&Q and ask for ICESTORM 5), I think this would all come to about £9 quid if that!. Then pop into Homebase and get a foam roller (dont think B&Q do foam rollers or you could just paint it with a paintbrush but a roller is easier.. £2.99 roughly).
Take it home, paint the MDF with ICESTORM 5 paint

and leave for about 2 hours + until it drys. Then point your projector at the small grey screen (preferably 40" if it will fit in your car) and then watch in envy at better blacks and much better contrast

.
If you like what you see then you can get B&Q to deliver a larger piece of MDF (I prefer 11mm thickness but some people buy 6mm or so) max size 8 foot by 4 foot which is about £11 I think, along with some more ICESTORM 5 paint if you only bought a small pot.
I think they charge about £10 for delivery but even then we're talking about total cost of £25 quid for a 8 foot by 4 foot grey screen!. B&Q will also cut it to any size you want so if you want it 7 foot by 4 foot then they will cut it for you aswell. You can make it look nicer in the living room by fixing it properly to the wall and surrounding it with black velvet (which also helps with lightspill for a 4:3 projector when showing 16:9 films.
Home Cinema does'nt have to cost a fortune
James