Good question vibha, and I
Good question vibha, and I won’t pretend to have the definitive answer, but the opinion around the road.cc office is that you could switch to seven speed and that you should just be able to squeeze that in to the same space occupied by your five speed freewheel.
Or you could try these guys – they list three five speed freewheels on there website – the closest they have to your current ratios though is a 16-20.
http://www.sjscycles.co.uk/category-Freewheels–5-Speed-77.htm