ABS light problem and brakes

[jt_uk]

Hi, I have been having problems with my Isuzu Trooper 3.1 Turbo Diesel LWB 1995 (it's actually a UK version called a Vauxhall Monterey) since the workshop took the hubs off to adjust the wheel bearings on the front wheels.

The brakes have been terrible, not responsive at all. However, after half a mile or so the ABS dashboard light comes on and the brakes return to normal. The mechanics had to move the ABS sensor whilst adjusting the wheel bearings but they noticed the ABS light coming on and have run diagnostics that prove it shouldn't be coming on.

Any clues as to what may be causing this is greatly appreciated!