Actually not a bad guess, Barry...'cuz that's the most common reason for sender failure. Waste contains animal fats, which build up on the sender(s)...effectively "insulating" it. So it either thinks the tank is full or empty, depending on whether the lights never come on or never go out.
If you're faithful about pumping out your tank at the end of every weekend, waste can also dry on the senders while the boat sits during the week, creating a hard "mud" buildup. Or it can be a combination of fats and waste.
It's sometimes possible to clean the senders by pumping out and THOROUGHLY rinsing out the tank, then filling the tank with water and about a gallon of low-suds laundry detergent (not any that include bleach and/or fabric softeners, just detergent)...go out and tack a lot (if you're a sailboat) or do whatever you can in a powerboat to agitate the solution...then let it sit for a few days before pumping out and rinsing out again. If that doesn't work, you'll have to pull out the sending unit to clean it. The first method may be a lot more work, but IMO ANYthing beats having to pull it out and clean it by hand.
This problem can usually be prevented by thoroughly flushing out the tank 2-3x/year...something that should be done anyway to prevents sludge buildup in the tank that can clog up the tank discharge macerator.
However, the best solution is a new gauge...the best value on the market IMO is the Snake River Electronics AcuGage, about £70 (if I've converted correctly from USD)for the single tank system...only about £15 more for a system that'll handle up to 4 tanks. The sender goes on the OUTside the tank, so it never comes in contact with the tank contents. I dunno whether it's sold in the UK, but the whole thing weighs so little that postage from the US is minimal. You might want to check it out. Their website is here: http://www.snake-river.org/ These folks have the best prices: http://www.boatfix.com/elec/acugage.asp