Thanks for your suggestion, bios was already up to date, but drive not, so updating to latest firmware seems to have solved the problem.
Link is now at 6Gb/s, here is a test:
Sequential Read : 493.138 MB/s
Sequential Write : 270.356 MB/s
Random Read 512KB : 398.744 MB/s
Random Write 512KB : 268.910 MB/s
Random Read 4KB (QD=1) : 20.551 MB/s [ 5017.4 IOPS]
Random Write 4KB (QD=1) : 51.009 MB/s [ 12453.3 IOPS]
Random Read 4KB (QD=32) : 188.841 MB/s [ 46103.7 IOPS]
Random Write 4KB (QD=32) : 219.028 MB/s [ 53473.6 IOPS]
Test : 1000 MB [C: 8.4% (20.1/238.4 GB)] (x5)
Date : 2014/06/20 19:54:54
OS : Windows 7 Ultimate SP1 [6.1 Build 7601] (x64)
mSATA 256GB CRUCIAL 20-06-2014
I suppose this is now ok? the average speeds i should be getting for sata 3?