The gas flame in the oven of my leisure classic 90 starts on tick over but than does not flare up to full flame like it should

It sounds like it is waiting to be Programmed for a start time. Try turning off the power to it for 5 minutes..

https://www.google.com/search?q=leisure+classic+90+oven+timer+instructions

This question also edited for clarity. 'make model (what it is)' first. Proper names with capital letters too. :>D

It needs a new thermostat. this has to be done by a registered gas technician, so call for service.

Sounds like the front burner gas jet holes are either blocked in places or the gas regulation (air/gas) mix to that burner needs adjusting. Try removing the hub where the gas burns from and cleaning each hole with something like a paperclip and then blowing through it to expel any particals.

It could be the flame failure device our Cannon had the same problem

Where can I purchase an Xenon II Instructions book?

I had a similar problem when my new Leisure cooker was installed last December. The burners would light when I held the knobs in but when I let go the flame would go out. The engineer took off each of the burner knobs and stuck a small wad of aluminium foil between the inside of the knob and the contact on the front of the cooker - he said it's a design fault and the gap is just a bit too large for contact to be made reliably. This fixed the problem and I've had no trouble since.

the flame safety device need replacing (FFD)... this lets the full gas through when flame is sensed... safety 1st as a gas engineer will be required..

Faulty FFD's ( flame failure devices) once the flame is established it allows the full flow of gas to the burner... must be a gas-safe engineer to replace part..

Replace the ignitor