This was the subject of similar posting last week for a 2.8 engine.
By coincidence I had the cam belt on my 2.3 JTD changed last week. The job is listed in the garage data as taking just under 3 hours, and the cost of the belt kit CW cam and tensioner is around £132. So if you allow say £50/ hr garage costs, and most garages charge a higher rate, you can expect the job to cost around £285 + VAT. So the quote from John is about right.
I had a service and MoT done as well as the cambelt and my final bill was £532!!