Sorry about your motorbike, and sorry you have to pass UK MOT with a 17+ year old car. That's a challenge in and of itself. You guys have some serious regulations.
Something that MB Doc hasn't mentioned is the contact ring behind the steering wheel. I'm not an expert, but I do have a 190E and the ABS light flickered on and off, and sometimes stayed on as I drove it. This car sat in a garage for five years without having been driven (it's my father's out on loan to me). I know that flaky connections can be flaky for a while and then just go out after a certain point. You could try disconnecting the battery, removing the steering wheel, and cleaning the contact ring. Might be as MB Doc suggested and probably is, but checking this is cheapest. Good Luck

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

Pull the bulb...

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1989 300E 144K

OK. I have checked all pins in ecu and relay connections on ABS unit. OVP is still fine. Took the car for a spin and tried several emergency stops. ABS is working but ABS light is still on.
I checked the front sensors with an ohmeter and the left one is reading 987 ohms and the right one is 976 ohms. It is getting dark so didnt check rear sensor. I presume they were the sensors........the connections looked like large tv aerial connectors....one either side of the car mounted by plastic clips!
I checked battery is charging and got a reading of 14.05 volts
Any ideas what else it could be?