This has been covered in some depth before a search will bring up good info.
My experience is here along with other people's experience with DIY and professional fitting.
http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=10068&hilit=Secondary+Glazing+AMc&start=30#p136640
I got a supply only price of £1300 from http://www.secondaryglazing.com/ but went for a professional fitted set from Everest for just over £1500 + a 15 year guaranty after a bit of haggling and negotiating.