i wouldnt recommend buying a ficus or umberella plant from b&q or homebase as they tend to use pesticides on their plants.
Also its just starting to come into the season for these plants to be shelved in gardening centres.
A good place to look is a notcutts.
Also ebay have a couple of ficus benjamina for sale at the moment.
Have you thought of alternative plants?
Pothos or Dracena are small plants, which should be ideal for the size you require.
If all else fails and you feel you have to purchase a plant from b&q i suggest you give the plant a good wipe over from leaf to root.
Hope this helps