I was down at a garden centre today and there were lots of flowers and young vegetable plants being sold in polystyrene moulds, 12 or 24 slots per tray. I bought a couple for the moulds!
The firm is Styropack (www.styropack.co.uk) and you can get 12 or 24 slot trays for £1.50 odd inc VAT + delivery, mimimum order size 60 units.
There are clearly other products based on the horticulture pdf document, but details are not available at the online shop.
Clearly you would not make a fortune being a reseller at Homeacres, but I would certainly have bought a dozen from you when attending a course.
I shall speak with local folks at the launch event for Hillingdon in Bloom in April to see if enough interest exists to place an order.