Went to a farmhouse restaurant up in the Jura mountains this evening and there was a purple buddleia full of them. Will post a picture tomorrow.
You can buy blueing agent (it contains aluminium sulphate) for hydrangeas in any good garden centre. Follow the advice on the packet/tin.
Most probably the sunlight/UV. It depends on the plastic, polypropylene disintegrates quite quickly, even though the makers say it won't.
Well done you! Amazing what a couple of old doors can be used for.
That is fantastic Birdy, can't wait to read the rest! You've truly hit the nail on the head!
Ooops you beat me to it!
Good luck Howard, let us know how you get on!
This might be useful too.
I found this on Google
I had one for several years. It lived in a pot and was cut down to about 12 inches in late autumn and stored in a cool dry place. Around March/April it would sprout again and was kept outside in a sunny place all summer and was covered in flowers. The bees loved it.