I have forborne to comment on this year’s Summer Exhibition which has gradually been taking shape over the last few weeks, making its long progress through the selection to the hang to yesterday’s Non-members Varnishing Day.   Today I was walked through the exhibition by Michael Craig-Martin, the co-ordinator (or Chief Hanger as he is sometimes still called) and have realised what an achievement it is, beginning with the multi-coloured staircase by Jim Lambie which transforms the experience of arrival.   Each of the galleries is more clearly differentiated according to who hung it and Architecture is given its own fine space by being in the Large Weston Room.   It feels more logical, as well as more exciting, partly due to the choice of bright yellow as wall colour for the Vestibule and brighter pink for Gallery 3.