[B][I]Year old room thermostat has packed up already???[/I][/B]

I had a new boiler fitted 13 months ago by a registered Gas Safe engineer - it's had its annual service done within the first year of instalment and all was working fine, until today when the room thermostat, fitted new at the same time as the boiler, has now stopped working. An engineer from Homeserve came out and confirmed this and advised me to get the original engineer who fitted it to take a look. He has agreed to come over on Thursday this week. His invoice said that all work is guaranteed for a year, but where do I stand with the repair or replacement of the thermostat - surely it should last longer than a year? The old one that he removed, was 14 years old and had never gone wrong? Any advice as to how I can deal with this when he comes over - I don't want to be fobbed off with buying a new one, after such a short time - bearing in mind, it was not used at all during the summer months, so in effect has only been used for about 5/6 months.