There are many myths about how ECU work is carried out, but in fact its not rocket science and once you remove the smoke and mirrors its quite straight forward.

There are a number of different ways to produce more BHP and torque from your Engine ECU, be it Petrol engine tuning or Diesel tuning, these include, but are not limited to. 

Chip replacement (ECU chipping, car chipping, chip tuning)
Flash reprogramming (Super Chips, Star chips etc) 
Piggy back ECU modules. 
Interface ECU modules
Custom ECU remapping
Live ECU reprogramming

S-TYPE 2.7 TDI £475 plus vat
Standard Performance 200 BHP 314lb ft torque 
Re-map 268 BHP 394lb ft torque

X-TYPE 2.0d  £395 plus vat
Standard Performance 139 BHP 212lb ft torque 
Re-map 169 BHP 260lb ft torque

X-TYPE 2.2d  £415 plus vat
Standard Performance 152 BHP 182lb ft torque 
Re-map 186BHP 291lb ft torque
X-Type Lowering Kits 30mm lower all round £485 plus vat
XK8/XKR Lowering Kits 25mm lower all round £495
XJ8/XK8 Full Stainless Steel Exhausts £695
Leaping Cats £70 plus vat
S-Type R Mesh Grille £240 plus vat  
S-Type XS Full Body Conversions, Colour Coded Front and Rear Apron, Side Skirts, Front and Rear Mesh Inserts Fog Surround. £1795 plus vat.