Hi,
Since you say the UK, is this a dedicated rear-bagging machine? The bit of info I find is that it uses a 1/2"(13mm) x 112"(2845mm) belt for ground drive. The center-discharge rear bagging deck I find uses a AA hex belt which is the rough equivalent of 1/2" and is 105"(2667mm) in length.
Advise if more info will change my answer. Most UK mowers are dedicated rear baggers from info I have seen.
Peace,
Clarke
Check these links:
http://www.vbeltsupply.com/ecommerce/double-angled-belt/aa
For AA, and
http://www.vbeltsupply.com/ecommerce/kevlar-corded/4lk
For 4LK1120, or
http://www.vbeltsupply.com/ecommerce/metric-wedge-wrapped-belts/spa
For SPA2850