My 331 Kalevala had a water tank sender failure. By now you will have located the tank, under the carpet beneath the saloon floor, just in front of the galley and the top of the sender, along with the wires that provide its power.
I bought a new sender from Wema. They were very helpful (01404 881810) and offered either one of two replacements, both cheaper than the original. One giving a precise reading, as per the broken one and the other providing stepped readings, which I felt was all that was required, at a cost of £100.80
Easy to fit and has worked faultlessly for 2 years.