H Smith Plastics

https://www.plantcell.co.uk/?s=root+trainers&post_type=product

Seed trays and inserts, multi-cell packs and square plant pots all made from 100% recycled plastic, at our factory in Essex.  H. Smith Plastics was established in 1963 to design, manufacture and supply plastic seed trays with inserts and plant pots for commercial growers, allotment societies and gardeners.

We also supply round plant pots and pot carrying trays manufactured by Modiform and Teku. Our pots and trays are recyclable and advances have been made to make black plastic recyclable for kerbside collection. 

Littlefields Farm

https://www.littlefieldsfarm.com/maxi-rootrainers?

The Maxi Rootrainers™ is the largest modular Rootrainer in the range, perfect for broadleaved trees grown on a long cycle to larger salable sizes and for plants destined for dry sites. Please Note: Rigid trays are an essential part of the system. 

40 cells/tray (10 books), Dimensions 44.5cm x 27.5cm x 13cm, Volume 350cc.

DT Brown

https://www.dtbrownseeds.co.uk/Garden-Essentials-1/Sowing-and-Propagation/Rootrainers-Deep.html

Rootrainers provide the perfect start for all seeds, seedlings, plugs & cuttings, especially those that require deep root runs or are sensitive to root disturbance. Rootrainers encourage vigorous and strong root formation - the roots grow straight avoiding root balls and pot-bound plants. The reusable, modular 32 cell planting system with holding tray and propagating lid allows plants to be spaced without repotting.