An update for anyone who may be interested.
Styropack were very helpful and Kevin and I collected some bags of these rigid polystyrene 24 cell trays today (see pictures in earlier post above).
They come in a minimum order of 60 trays – if you collect in person.
Cost is 24p per tray plus VAT ie £14.40 + VAT for 60 trays.
They will post if you prefer, but I think the minimum order is 480 trays by post, and I believe the unit cost is higher to include postage.
We dealt with Styropack in Ford, near Arundel West Sussex 001903 722811.
They also have depots in Blackburn and Aberdeen, who presumably can offer the same.
http://www.styropack.co.uk/
Good luck!