It sounds like it is a C4 model. These have a mechanical float level on the front which is moved by a magnet on a float in the holding tank. The float in the tank may be stuck or have fallen off or the indicator on the front could be stuck. These bits are available although the colour choice on the indicator may be a bit restricted. I haven't done it but i think foitting the float in the tank may ba a bit of a hassle. Bob has guided you to the repair instruction which have photos - although they are of tanks before assembly and all nice and clean.
The outside indicator should cost about £11 plus p&p and VAT and the float arm should be £4 ish plus p&p and VAT.
William, April 2013
