Leisure rangemaster 110 fan oven element replaced but still does not get hot

There are 3 possibilities:

Faulty thermostat
Faulty wiring
Faulty oven control switch

Thermostats fail more often in the closed position which means your oven will overheat so I'd test the wiring first. You'll need an Ohm meter to do this. TURN THE POWER OFF FIRST, expose the element and switch then trace the wires from the live and neutral terminals on the element back to the switch. If you can't trace them, make a note of their colours (at the element end) and test for continuity/resistance with wires of the same diameter and colour at the switch. You should get a reading of zero ohms if you have good continuity.

The easiest way to check the switch is visually. Make a note of the wiring colours and their positions on the switch terminals, disconnect them and remove the switch. See if anything looks broken or the

Some thermostats can be disassembled and inspected, but I don't recommend it unless you know what you're doing. The gap between the thermostat switch contacts needs to be precise or the oven wont heat to the correct temperate.

If you can establish that the wiring and switch are OK, try replacing the thermostat.

In this case then, you have a fault with either the thermistor, and/or the actual control switch, itself.
http://kitchen.manualsonline.com/manuals/mfg/rangemaster/professional_110_dual_fuel_u1_216_02.html
Your user manual is above.
To repair you would need a service manual, and a meter, to troubleshoot and test the circuit, and order the correct parts number in this manual.
Otherwise it maybe best to call a repairman, and get it looked at by a professional, in case it is something more?