Using the corex signboard from the for sale sign I rescued from the road & traffic I cut panels to fit into the back of the space saver greenhouse and covered them in aluminium foil shiny side out, and have installed into the greenhouse.
Because the space saver has shelves at 290mm centres, the lower shelves don't get an awful lot of light. I have probably made the problem worse by adding small selves to the front section. Where the greenhouse is located means that it gets full sun in the morning and by the afternoon the apple trees provide shade.
I have made some reflective panels that can be added just behind the front shelf directly behind some the small propagators. I have a couple of more boards at the allotment I can bring home to complete a foil back panel for the whole greenhouse.
I now just need it to warm up so I can start growing seedlings.