Joe, if you can make contact with them, try to get the spec list on it? also ask for another price as in for supply only. The £185 is for supply and fit.
It would work on a remote control, but bare in mind, anyone who buy's a remote control, look at the range and then deduct at least a third from the range...ie if it say's it has a range of 150 ft then in reality its only 100 ft.
also if anyone already uses a remote you could install a 2 chanel system to operate your original system and on the other chanel you can have the heater.
Now for the downer :

I can't see the spec on the link, but just looking at the size of the cable (red) and the inline fuse, also knowing its an instantanious water heater, the current consumption from a battery is going to be huge :

and unless you have some way to keep your battery topped up and quickly its not going to work. it might be ok like Alex said for the odd bird poo situation, but its a non runner for using it lots during the day.
Sorry if I have put a downer on it, but I don't want to see money going to waste.