On a rather dreary and wet day, two dozen Blue Badge Tourist Guides met in the foyer of the National Theatre for a "behind the scenes" tour. We were split in two groups, all dressed in fetching high viz jackets. Even before we set off our little band was buzzing with excitement, as we were promised a goody bag full of information leaflets on our way out.

I’ve taken my cue from an erstwhile neighbour – Alfred Hitchcock was born just up the road in Leytonstone – and I’ve moved to the rear window. I’m in search of the colour purple. But instead of the lilac that coyly shows its first blush this time of year through the satin-white of the magnolia tree and the billowing chartreuse of the willow, I’m getting a Phoenician purple that would have turned Queen Elizabeth I, who banned the royal colour from her court, apoplectic.