What parts might I need for my Vaillant 282 and where can I get them from?

I have a vaillant 282 combi boiler and the radiators are running cool although the boiler shows about 30 degrees. The green adjustment knob on the boiler no longer engages with the control behind it and I was advised to order a new one, although I'm no longer convinced that this is the whole problem. Could anyone advise. I have an engineer from the (corgi registered) installers booked for 20th Nov and would like to obtain parts before then to avoid a 2nd visit