Wifey is beginning to think that our gas oven isn't working as it
should, ie, things are taking too long to cook. She's had some potatoes
in there (she's making 'Slimming World chips'), on gas mark 9, for
almost 50 minutes now and they're almost, but not quite, ready yet. On
the other hand, a joint of beef roasts in what she considers 'normal'
time, although she does come from the 'slow cook for long time' school
of thought on that.

How does one go about checking correct operation of gas oven and making
sure that 'gas mark 9' really is gas mark 9 - and what do you do if it
isn't working as it should?