Newatt Heater Issues

[betta fish]

Hi all, I'm currently having a problem with one of my Polymer Newatt heaters that I hope someone can help me with, it has been working fine for the past 3 years I have had it, keeping the tank at a steady 23 degrees but just yesterday after my weekly water change it stopped working, I never unplug it when I do the change as it has an auto shut off unlike glass heaters so I have never needed to and only a small amount of the top of the heater is exposed, I noticed the thermo said 20 this morning and sure enough the heater failed to click on when I raised the temp on the dial, I removed it and replaced it with a spare glass one for now but have since replaced the fuse in case this was why and submerged in a bucket and switched on and was surprised to see bubbles flowing quickly out the top where the cable goes into the heater?! almost like it was plugged into an airline lol! not sure what this means but I immediately switched it off and the bubbles stopped straight away, anyone know what may have caused this? for those of you unfamiliar with the heater it is this one:http/www.123aquatics.co.uk/products/aquarium/aquarium-heaters/newatt-heaterstat-958.aspx