Faulty ABS? Daewoo Lanos, 51 plate, 1.4 SE?

I have a Daewoo Lanos 51 plate 1.4 SE and I am having a problem with my ABS.

My warning light comes on and off at different times for no particular reason. Sometimes, after starting the engine (after the initial period where the other dashboard lights are on shortly after turning the engine over) it will be of and remain off for a journey. Sometimes it will be off then come on and other times it will just be on.

Also, when the light is on, if I turn the engine off then back on, sometimes it will go off.

When it's off, the ABS works as normal, when it's on, it goes back to normal, non ABS breaking.

I took it for a diagnostic check and was told the below was wrong

RF Speed Sensor Malfunction, Parts needed: OSF Sensor and Bracket @£100 + VAT
NS and OS CV Joints ABS rings split @£239.09 each + VAT
Labour 2 Hours @£60 +VAT per hour
Total (inc VAT) 820.36

For all I know, this could be true but it seems like a lot to go wrong at the same time and, if all that were wrong, would it really be working sometimes and not others? I had asked if, based on the same information I have put above, it could be a faulty sensor to which I was told no.

Would anyone be able to give me some advice on what they believe it may be, or if it really could be what was diagnosed.

Thank you