caple cr9200 not heating up properly

Hi, i have a caple cr9200 duel fuel range cooker. the main fan oven no longer heats as well as it used to, if i set the temperature to 200 it will struggle up to about 160/170 but never seems to get above that. the thermostat light stays lit and doesn’t go out unless i turn the temp down and then it will go out at around 160 and maintain that temp. Have tried all the different functions ie convection, fan etc and it makes no difference, thanks in anticipation Mark