Fitting a filter has always been advised as 'best practice' and my understanding is that it became part of Part L Building Regulations last summer and was a requirement. Might be best talking to the boiler manufacturer just to check they will honour the warranty etc without a filter being installed.
From the 4000 installation manual, a bit long winded but they say if you don't follow guidelines it could 'invalidate the guarantee' and that a filter is 'recommended'.
I think it covers them for turning up and if the system isn't clean and has caused the failure they can refuse repair.