I have no gas going to the oven on my Leisure Victoriana NL and have been told it is the timer as everything else works ? Can you tell me how to set the timer ?

SOURCE: leisure rangemaster 55 gas cooker

It sounds exactly the same as I am experiencing on my oven of the same model. My theory is that the full flame gas is controlled by a thermostat, and that this has broken hence why the oven only stays on preheat. This will be a matter of a replacement part. Most likely the thermostat is a bi-metal strip that, because it has heated and cooled so many times in it's life it has simply finally given up. Try searching online for the correct part and get a corgi registered fitter or another suitably qualified fitter to come and replace it.