There's a zoomable parts diagram for the very similar 28168 deck on a UK parts site HERE.

It shows the relation of the jockey arm [No. 11] to the spring [No. 12 in this diagram], and that double-nutted bolt the spring hooks on to, a fair bit better by showing the spring twice. Same spring part number as grumpy has stated.

This will likely be a fairly hefty spring, so I would also recommend trying to find the Rover part.

Cheers,
Gadge

"ODK Mods can explain it to you, but they can't understand it for you..."

"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."