CATS have a very good reputation.
The FI course is run by Bob Moore not Stuart, but he is behind all the workings of it though and as another poster wrote too, he is in the stages of becoming FI instructor too.
I just get the feeling it's probably better if you did the ATPLs with them first though as the schooling seems to start from day one.
The £4999 price sounds right for the C150s that they do it in