Red STOP light coming on with message 'Fault with injection' and a bleeping noise. Hooked up to computer and advised to change heater plugs, which was done and still happening but computer is confirming nothing wrong.Service light is still on though! Anybody come across this before?Serious ansers only please! Thanks

Have you taken the vehicle to a certified Diesel Engineer? If not you would be advised to do so. This kind of fault cannot be diagnosed withought hands on checking.
If the engineer comes up with an answer and needs parts give http://flag-diesel.rivousel.com a try, they are very reasonable in price with high quality.

Did you change all of the heater plugs or just the faulty one? Also check the EGR valve as this can block/stick