a past posting reference
it is a submersible heater - see the paste from a post a few years ago:
Andrew looks like he's up against the limited capacity of the Jobo for tempering enough water for washing and preheating enough chemistry for a long session. I've seen the prices on the Jobo TBE, and had problems with spending that much on a plastic tub and heating element.
Given that, for an auxiliary tempering bath I use an aquarium pump and a submersible heater with thermostat, DevTec Model TC-750 made for photography by Doran Enterprises,
www.doranet.com, and available from B&H Photovideo and others (
http://tinyurl.com/6tjfk). I put them in a shallow plastic storage box with closed cell foam on the bottom and sides. You could also use an ice chest/cooler/eski for a bit more efficiency. I know some people use aquarium heaters, and they could be less expensive, but I went with the US$110 purpose built unit because it's designed to withstand exposure to photochemicals. This arrangement certainly helps when the Jobo capacity isn't enough for wash water, and holds temperature to a fraction of a degree very well. Perhaps there's something similar in the UK market, or an aquarium heater would serve. I suspect the 750W DevTec might have a larger heating capacity than small aquarium heaters. It is designed to work with color processing temperatures, although I've only used it at 75F and lower.
Sorry, but I don't know if this is distributed in the UK. It certainly looks like it's still made in the US