depends on what make and model of fire it is, if its some �100 quid crap from b and q then may be worth replacing, if its a decent �400 fire from Valor then is worth repairing.
what make and model of fire is it?
and the gas valve is not like one you have on a boiler, its just a small magnetic valve unit that only cost a few quid.
most problems like this are not down to the thermocouple at all, 9 times out of 10 it can be cured by simply having the fire serviced, we only charge �40 for a service.
if your in the birmingham area let me know.
by the way, if you haven't called out a corgi registered gas fitter yet then who replaced the thermocouple??