Docket, we've got a one of these for ds and they're fab. Very lightweight, fold down easily for holidays etc, and fit to any chair. Had it since he was a little over 6 months, now nearly three and still going strong, and will hopefully be used for number 2 when she arrives.
We have the mothercare one that clips to the side of the table & it is ideal.
It was bought for ds to use when eating at my parents house, but we wish we had got one of these for our house, rather than having a bulky highchair.
It has a full harness & is great for travelling, as it takes up very little space.
I think cost around £25.