I bought my 98 300 tdi es disco from a friend a few weeks ago with a full mot. He told me the garage had had to mess with the abs to get the light to go out to pass the mot. Now I have noticed the light goes out but sometimes comes back on while driving.The brakes seem to work the same as before so is it ok to drive around with the light on or does it need sorting out soon ?

The brake will function as brakes do, it's just that u may have lost the ABS function therefore your wheels could lock up, have you tested to see if they do lock, but then it may be a loose wiring connection, anyway it should be sorted out sooner than later.