Price to fix vaillent boiler

thor

My mother is having a problem with her vaillant ecomax 824e and had been toying with getting a subsidised new boiler. We contacted one of the companies offering this service and after the first survey she was informed that it will cost over £900 for a 'free boiler' plus whatever extra the 2nd installation engineer survey reports. She does not have this kind of money so instead opted to get it repaired. I got an engineer round to see how much it would cost. The boiler still produces hot water and warms the radiators ok but it is making a knocking/ticking noise and we have turned it off as a precaution.
The engineer says that it is being caused by a diverter valve which will cost a total of £160 to replace. Does this sound like the solution for the problem and how fair is the price?