on the Pano there are currently 7 of those towers under construction:
Broadgate - 165m
Pan Peninsula towers- 147m / 121m
Arrowhead Quay - 114m
Crossharbour - 131m
Marsh Wall towers - 140m / 98m
Castle House 147m will soon begin construction by the end of the year.
Leadenhall 225m - will as Jeff says begin construction next year and the building it will replace is currently being demolished.
Demolition of old buildings in preperation for new towers is currently taking place at
London Bridge Tower - 310m
The Pinnacle - 288m
20 Fenchurch Street - 160m
100 Middlesex Street - 105m
Riverside South towers - 236m / 189m (ground works)
Heron Tower 242m
100 Middlesex Street will certainly begin construction, though obviously with the other 5 market conditions and financing will decide, though London Bridge Tower does have an office pre-let and and a hotel confirmed.
a current shot showing the Pan Peninsula towers and a load of mid-rises under construction at Canary wharf