Oven does not seem to get hot enough

[Mandy2003]

I've just tried cooking a meal on my Dad's new freestanding gas cooker (New World Victoriana). Something which should have (according to the instructions on the packet and when I've cooked it at home) taken 20 minutes at Gas 6 was still tepid when the time was up. Eventually after turning up the gas to max it was cooked after another 25 minutes.

I suppose I will have to get an oven thermometer to check it, but what's the usual cause of this happening? The flames at the base of the oven at the back look longer and yellower than mine used to when I had a gas oven, if that's a clue!

the flames should be blue, and should diminish to little pimples when it reaches temperature.

It's not on bottled gas, is it?

No, mains gas PigletJohn

TirednessKills - I was comparing it to the gas oven I had before I went electric.

if it's new and not reaching the temperature set (check it with a thermometer) then I think a call under guarantee is on the cards.