Hey Buster
Looks good,I dont know if in the UK you can get FOAM INSULATION BROAD,
But here it comes in two thickest 12.7mm and 19.05mm here.It has a sivler foil on one side and a dull sivler foil on the other side.Great stuff you can cut it with a knife to the size you need.The sheet come in 1.21m x 2.43m and over here it around $8.00 to $12.00 a sheet.I lined my closet with it , walls&cealing too, and cut in a aircondition vent in the cealing.It drop the temps. down some too.Use foil tape to seam the broad together.It also stops light too.Great stuff to work with.Your garden looks great,keep us posted.