I have one of these and when the work they are wonderful, but when they don't they do tend to drive you insane! (this is normal for an old ford)

can you give us a list of the parts you have replaced please as it is unlikely that you will have 2 duffer parts (but is possible) also the CPS sensor can cause a variety of issues so is worth looking at, another problems is dodgy connections anywhere in the engine loom (this one is the same for all cars) so a bit of effort put into checking and cleaning ALL of the connections is well worth while.

While I use other people for my toy now Mr Bayjoo at the Grove garage is in your neck of the woods and realy does know Fiesta RS Turbo engines inside out (including the managment)! they are well worth a visit!

Fuel pressure regulators are all worth looking at for this sort of problem. Another really silly one is is AIR leaks especially between the turbo and the manifold! - had both of these in the past too!

PM me if you have any questions and I'll try to help.

ALSO PLEASE CHECK YOUR GREEN RELAY in the wiring loom behind the fuses where drives right knees is. This is a Well know problems that can cause disaster as it stop the FAN working..... and melts the loom at the same time!