Here is a further update.
In September I raised a complaint with Transport Focus. They sided with Virgin Trains and said that I was only entitled to compensation for the ticket relating to the delayed train. I escalated this to a more senior person for review, but the reply was the same. What I found disappointing in the reply wasn't the result but it's supineness. It looks as though they contacted Virgin Trains and just accepted what they said. I was expecting at least an explanation quoting evidence as to how and why they reached their conclusion.
This was the reply from Transport Focus:
"The 0830 service that you took from Birmingham New Street to London Euston was terminated early due to delays and you have been compensated to the value of 100% of the value of this ticket cost - £17. I appreciate that you arrived over three hours late in to Euston, which had a knock-on effect on your journey to Kew Gardens. However, as you did not hold a ‘through’ ticket, but rather, were travelling on a combination of a single from New Street to Euston and then an Oyster card from London to Kew Garden, you are entitled to compensation based on the single ticket for the affected journey only (New Street to Euston).
In order to be eligible for compensation for your full journey between Northfield and Kew Gardens, you would have had to purchase a ‘Single’ ticket between these two stations rather than the combination of tickets you actually purchased, as detailed above.
As my colleague <xxxxx> has already mentioned, if a combination of tickets is used for a continuous journey, compensation applies only to the individual ticket covering the section of that journey where disruption occurs and will be calculated on the basis of the cost of the (Single or Return) ticket for that section of the journey and the length of delay to the passenger."
I thank all of you who contributed to the discussion. I personally am bemused that the Railway can't put together a concise and accurate form of words that I as a fare-paying passenger could understand and know exactly what I was entitled to if things went wrong. Terms like 'journey', 'ticket' and 'train' are not defined and are sometimes used as though they are interchangeable.
I am surprised that the local journey from Kings Norton to New St was not included in the delay-repay payment as I thought a split-ticket journey was still a journey. If, say, the train from Kings Norton had been late and I missed the Euston train I would have expected the railway to honour the Euston ticket at no extra charge to myself.
I agree with those that have said that introducing the 'return' part as part of the delay compensation is only sensible if you decide to abandon your journey. To differentiate the severity of the delay by multiples of the single journey makes more sense (Eg 50% for 30-59 minutes delay, 100% for 60-119 minutes and 200% for 120 minutes or more).
The moral issue of whether you should claim at all in the event of a delay due to suicide is an interesting one. Even if I didn't want to claim, Virgin insisted I be paid automatically. This is OK as my intention was to donate the whole of any compensation to charity.
I do not intend to pursue the claim any further, but I will still be interested in your comments.