Can a garage declare my extended warranty void due to a missed service even on a non service related claim?

I took out extra warranty when I bought my car. The head Gasket went just under a year ago and was replaced under warranty. The car has now developed the same problem. The garage replaced the Thermostat but said the Warranty was not valid because I had not had the car serviced. (It did have a full oil change with the new head). I had to pay for the replaced thermostat and the garage gave the car back to me saying that it seemed fine to them and was not overheating but that I should keep an eye on it. Within 2 days the car was losing power and billowing smoke. It has now completely died and I fear the Head may have gone again. (Still within the 12 month period since it's replacement.) Surely the Head Gasket should still be under it's own warranty. Also this would not have been covered at a normal service so how could a service have helped. Are they just trying it on? Should I be covered for this? Should they have replaced the Thermostat as standard procedure when the Head went first?Can a garage declare my extended warranty void due to a missed service even on a non service related claim?A lot of extended warranties are the same. Miss the service schedule by a day without calling them to tell them it will be done tomorrow, miss the mileage by a mile, and many of them are immediately void.