Why not get bold:
Get yourself a wall chaser, put a 30mm channel into the wall about 30mm deep, dig it out with a hammer and steel, coat the channel liberally with some PVA adhesive and stick in some 20 or 25mm conduit and drop your own cable down from upstairs. Connect into the backbox with a conduit connector (20mm).
Glue the conduit into the recess with a hot-glue gun. Slap in some base coat plaster leaving about a 2mm depth.
Finish with some finishing plaster when the base is dry, get yourself a proper plasterers steel. Rub over about a day later with a sander and you'll never know you were in there.
When we renovated our house, I stuck in about 30 cat5 outlets doing the above, and once you have got a couple under your belt, you'll be happy to do them wherever you like! A floodwired house is a happy house...
BW