I was in Homebase this afternoon, and they have a policy of giving away cardboard boxes - they have a big stack of them past the tills, and a sign saying 'please help us to recycle - help yourself to boxes' or something like that. They also had a load of large plastic pot trays, the kind that they keep arrays of potted plants in on the outdoor shelves. I bagged a few good deep ones (that I reckon will be fine to plant seedlings/plantlets directly into), and also a flat shallow tray, good for placing pots on.
This was the Reading branch, don't know if others do the same.
Also in Homebase:- Bags of compost for £1 each. If the bags are damaged they reduce them to £1 to get rid of them fast. You need to be quick off the mark to get hold of any and always ask a member of staff in the outdoor plants section if you can't see any.
Most of what winter bedding plants they had were half price, too. Got a tray of dianthus for £1.49. The violas looked quite good - but they were yellow, and I would prefer purple. Grasses were going cheap, too.
If that homebase was also had a load of large plastic pot trays, the kind that they keep arrays of potted plants in on the outdoor shelves.It will be great for that things.
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plastic training (http://www.mappinc.com)
Those dianthus I picked up end of season, which spent a hard winter outdoors still in their plastic tray/pots thingy being frozen, defrosted, and refrozen - are going great guns this summer! I do love plants that will put up with abuse and still flower like crazy. :)