As I've said, you WILL need to get the codes read, otherwise you're going to be chasing around changing this and changing that 'cos somebody says "that's what happened to mine" and trust me, that will get expensive.
The 3 amigos usually, but not always show if there's a problem with a hub/speed sensor, but there are four of them at around 80 quid a pop, so finding out which is the faulty one is pretty well essential, but there's also the possibility that it's a fault on the Shuttle Valve Switch Plate which is under the ABS modulator, not ACE as has been suggested. Even if you change the switch plate at the same time as doing the mod it'll cost about 40 quid.
There are some less usual causes, but only getting the codes read and the historical codes cleared after the work will sort things out.
As long as you know where you're going then it shouldn't break the bank.