Peugeot 206 speedometer not working?

A few weeks back I had a flat battery which was charged, engine started but I didn't go anywhere. The following day it wouln't start again so gave it a long charge and went out and then noticed the speedo wasn't working at all. Turned out the battery needed replacing and was told the speedo most probably needed a new sensor. Got the new sensor and was told it might take a mile or two until the speedo started working, which is what happened after five minutes it started to work.
The following week each day it would take a minute or two to start sometimes less, occasionally it would start straight away, some days not at all on the short trip to work (five minute trip). When it did start to work I noticed it was reading about five mph less than what I was doing.
Last few days it hasn't worked at all again, read about a fuse but reversing light works so it's not the fuse that controls both. I was told about a cog but the milometer works so it isn't that and every other dial works fine so has anyone any ideas how to solve the mystery?
Would a new cluster help?