Thanks for all of the replies, to answer all of Spark123's questions:
The cable will be chased into the wall, although in the loft cavity it may come into contact with the thermal insulation (glass fibre). It won't be grouped with any other cable. The ambient temperature is very unlikely to...
Thanks Daytona, the lights and fan are in a new bathroom and so will be inspected by someone from the building regs. Are there any safety issues in putting the 1mm cable within a 1.5mm circuit? I imagine doing this would automatically downgrade the whole circuit.
My lighting circuit consists of 1.5mm cable, I want to add an extractor fan which would share a switch with the room light but the fan states a maximum 1mm supply cable. Do I have to install the fan on a separate circuit?
Thanks