Timer on an emersion heater

I'd like to put a timer on my emersion heater, but they are very expensive. At the moment, my emersion simply has a switch with a heavy duty grey lead going in and the emersion lead coming out. Is it possible to put a standard UK 13amp plug on the emersion lead, and take out the switch and replace it with a standard plug socket? That way I can use a standard £5 timer and not have to spend £30 on a dedicated emersion timer. Thanks for your help. Chris