It could be that data in a critical sector has been corrupted and the drive is trying its best to read it--causing the clicking.
There is no way I would spend £800 to have someone "try" to recover my data (none I have is worth that much). This sounds like what a data-recovery company would charge to disassemble the drive, put the data platters in another good drive, and read the data. All this is done in a clean-room environment, and is therefore expensive.
As an alternative, I would suggest you give the program
SpinRite a try. Read some of the testimonials at the link above and see if you think it might help you. It is not cheap, but it has saved the data of many hard drives.
Best of luck,
WallyH
Note: I have NO financial interest in this company. I just know it is very reputable and successful.