Hi guys

hoping for some help, ok so driving along the other day and the cel came on, fault code 21, temperture sender? went out to it the next morning and the car takes ages to start and runs very rough for few mintues, fans also cut in as soon as the key is turned. im hoping its just the temperture sender needs changing but wondering weather this would be telling the car to start cold off choke which is why it takes so long and runs so crap?? seems to start fine when warm. any ideas on where this temperture sender is located or where i can get hold of a new one, near romford essex?? its a 93 import wrx.