Great Western Coffee Shop

I booked a return ticket from Exmouth to Bristol Temple Meads on the 11th of December 2012.
My outward train left Exmouth at 0645 but it was cancelled due to mechanical problems. Fortunately I checked before leaving home and therefore drove to Exeter St Davids, parking in a nearby car park and catching the train for which I had a ticket and reservation. I had to be in Bristol by 0845 and therefore catching a later train would not have been an option.
At Exeter I was advised to submit a claim form which I duly did.
Today, I received a letter stating that compensation was only paid for a delay of an hour or more. I was also sent a travel voucher equivalent to the cost of my car park charge.
Is this correct?

That is correct.  In fact, there is actually no obligation for any compensation to be provided in these circumstances as you still arrived on time.

An alternative to remember for the future is to get a refund on your original ticket from Exmouth (you are entitled to a full refund if your train is cancelled) and then buy a new one from Exeter.

If you had caught the next train from Exmouth and later XC connection would you have been over an hour late?  If not then you wouldn't be due compensation.  If you had an "Advance" ticket and your train was cancelled then you would be entitled to a refund if you decide not to travel or if you arrive at your destination an hour late - the issue is you still travelled on your booked train, so the gesture of the car park charge probably is correct as you made the decision (understandably) to drive to EXD.