Let's forget about all those rather silly cheap face-masks that some people use. If the mouth/nose bit gets bunged, then the dust simply comes in around the sides. In fact, even when not bunged-up, they never fit properly, and lots of unfiltered air creeps in under the edge.
The old "gas-mask" type are much better. Rubber/elastic straps for a good fit. Always go for throwaway separate filters too.
For you, I'd guess that dust in the eyes could well be just as much of a problem, so I'd go for a full-face mask. They are much more expensive though. Even more expensive (hundreds of pounds) are full-face with powered extraction (re-chargeable batteries).
Not wanting to spend that kind of money ... you should maybe go for full-face with throwaway filters (unpowered.)
http://www.screwfix.com/c/safety-workwear/dust-masks-respirators/cat850440#category=cat850440&page_size=20&page_start=0
The ones listed as "without filters" just means you buy the right type of filter separately and plug them in.