Many thanks for the words of advice to those who emailed me ---
In the mean time I have managed to obtain a finish reamer and the rifle is again working and shoots within 1 inch at 100 yards - so I am pretty happy!!
there appeared to be a minute fragment of steel* that had somehow become embedded (or adhered/welded?) to the tapered part of the chamber near the shoulder. It had withstood some serious scrubbing with a bronze brush. But without a boroscope it was impossible to see but the reamer cleaned up the chamber a treat.
* I know it is steel because I removed it from the reamer with a magnet
BTW It is in fact a Model M, but still very light and with a heck of a recoil! (have to be really careful of the set triggers)