Maplin is good... Will be surprisingly inexpensive in comparison to a spare parts supplier...
Take original one with you if you can - one of the staff might be able to find the best match or nearest equivalent - Unit of capacitance is the Farad (very very large) most capacitors are rated in Microfarads (uF) - They also have a Voltage rating - make sure your new capacitor is the same or more Volts - if less it may go Bang!
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.