Chiltern Railways replied very promptly to my complaint as follows ...
Dear Mr Old Yard Dog,
Thank you for getting in touch.
I am sorry that your train from South Ruislip to London Paddington was cancelled on Monday morning.
The train was cancelled because the driver of the train was unable to connect the train's radio to the GSMR radio network. This is the system that is used to communicate with the signaller (and is considered to be safety critical).
Since Monday, due to an identical fault occurring (for the same scheduled train to Paddington) on subsequent days, using different trains and with different drivers, we have been able to prove that the fault was with the local Network Rail radio mast and which only affect our once a day trains to and from Paddington. The resolution of the issue was taken very seriously (we do not like cancelling trains), to the extent that yesterday the train ran with two drivers and a member of our engineering team on board.
Since the beginning of the current timetable the train has been cancelled on three occasions:
Thursday 23rd February - Storm Doris (A strategic decision to ensure that we had as many drivers as possible at Marylebone and avoid the risk of the train being stranded at Paddington in the event of infrastructure problems)
Monday 27th February - GSMR Fault
Tuesday 28th February - GSMR Fault (this was when steps were taken to understand the root cause of the problem).
As your journey was delayed by more than 30 minutes, under the terms of our Passenger's Charter, you are entitled to apply for Delay Compensation as a result of the cancellation.
If you wish to apply for Compensation please submit an application via our web site. The Enquiry type is 'Delay Compensation'. If you travelled using an Oyster Card, please attach a copy of your Oyster journey history as an alternative to submitting a copy of your train ticket.
https://www.chilternrailways.co.uk/compensation
Alternatively, please reply to this message with a copy of evidence of travel.
The train ran to time today, so I do hope that you have the opportunity to use the rather unusual train service to Paddington sometime soon.
(I won't be asking for compensation)