It was twice yes, but only ever to the Newton Road bridge. West of there the original track is still in place, broken and warped as it is. The first relay was for WCML upgrade infrastructure trains, while the second one was to re-do it as a freight run-round.
It wasn't relaid as far as Swanbourne sidings, just as far as the Newton Road bridge. It's been relaid twice since then, and is mainly used as a run-round for freights and infrastructure trains. I don't know about access arrangements during those works though.
What do you want to know about the line? It's in use to run round trains as far as the bridge over the road from Bletchley to Newton Longville. There's a buffer there and the track after that is largely warped and broken, with long stretches removed by metal thieves. Swanbourne station is privately owned and very nicely restored. Winslow station is demolished entirely except for the platform faces, and the new Winslow station will be built further west by the Buckingham Road bridge. There were once extensive marshalling sidings east of Swanbourne and there's talk of putting a freight loop in here when the line is rebuilt.