Update on this.
If your hour counter has failed, there is a document with instructions on how to replace it with a stand-alone unit in the Downloads section.
If not, then I suggest some preventative measures.
Dismantle the tachometer. Lift the top of the white ribbon cable socket and detach the LCD.
If there are any salt crystals inside the unit, rinse them away with warm water followed by de-ionised water. Don't get water in the motor if you can help it.
Mop up any remaining water, and use an air blower to dry.
Put it in the airing cupboard over the winter.
In the spring, coat all connections with Contralube 770.
Attach a 1g bag of silica gel crystals inside the base, a glue gun works well.
Reassemble.
Cover all holes with a piece of insulating tape, or a blob of silicone sealant.