What the Yak did at NW seems clear from the accident report and the appended diagram, the latter being accessible via a link on the report on the AAIB website.
The report is
here .
The Yak came back from Little Gransden via west of Harlow, round the south of Harlow, along what might have been a RH base leg for 20, but extended that leg to the proximity of his house.
He then turned 180 left (away from NW), effectively joining a LH base leg for 20, before turning L again for 20. He came to the Cessna which was doing a (conventional) RH circuit and their turns coincided and collided. It was concluded that they met belly to belly. One or both might have steepened/tightened the turns in a last second attempt at avoiding the collision.
In the course of the Yak's progress he mentioned on the radio "initial" (whatever that means), in three transmissions it seems, and also dived down to about 100 feet near his house and climbed away. I have no idea what those manoeuvres have to do with a R&B (or VRIAB).