The fuel gauges and all your other engine gauges are made by VDO a Germany company, the good news is there is a very good supplier in Portsmouth, Furneaux Riddall, (general electrical suppliers to auto and marine world) Portsmouth 02392668621, they have been very helpful to me and other NA members.
It sounds like the sender unit has failed, (most likely the rheostat (variable resistor) has got saturated with fuel and has failed or the wiper arm is not making electrical contact.) another check to confirm this is to remove the wires from the sender and measure the resistance and see if it changes when the float is moved. You would expect a range of around 10 ohms to about 300 ohms, if you see no change in resistance then the sender is faulty. Depending on the detail design it may be possible to clean the rheostat but most likely you will need a new one.
Attached is a wiring diagram from my NC 35, it will be very similar to yours, also attached is a simplistic wiring diagram which may help.
Fuel, from picture its hard to tell, but it does look dirty, an indication would be the state of your fuel filters when you change them, ie if they are very dirty after a seasons use, as eventually they could become blocked. Some members have tried cleaning their tanks my making up a pump and filter system to circulate the fuel through filters and back into the tank, see Cat-A-Log Winter 2012 Number 62 “Hunting the Fuel Bug”. Maurice used a company called Express Lube (Chris Roos) 01444254115, they charged £350 + VAT to flush the tank, I believe Maurice was happy with them, also I think they put an extra inspection panel in top of tank.