Having only one lug bearing is a Very unhappy situation, both from an accuracy and engineering standpoint. Don't think lapping is a good idea at all in this case. Either use another action to start or find a bolt that at least bears somewhat better (and isn't flogged out on the front end) before proceding.
Captain Laidlerhas written extensively on fitting new bolts to No.4 rifles in this forum. I reckon the practice is the same w/ a SMLE. Don't know how late he reads this forum, but there's sure to be more detailed info coming.