This is due to engineers working on the tempoary o2 Mast that is in the car park on Osmaston Road near the old DRI.
Here is a photograph of the o2 mast that they are working on, its located in the car park of the old DRI.
Here is a map showing the o2 mast location.
Here is what your iPhone data indication looks like when you have lost 3G and its dropped down to a really poor GPRS connection :
o2 Sent me a text message on Monday "o2: Just to say, there's work planned around DE1 2QY over the next few days. it may affect your signal, but once it's done you'll have a faster smarter, stronger network, see more "http://s.o2.co.uk/Status/ByWHae"
But that link does not show any planned maintenance.
They also sent me this exact same message on 5th September with a different web address.
Here is the o2 Network Status checker : http://status.o2.co.uk/
On an iPhone it tells you your data connection method in the top/left of the screen. If you are on a wifi connection this this will hide the indicator.
The data connection method normally shows either "4G" "3G" "E" "o" or nothing.
3G vanished a few days ago and I can only get GPRS now (indicated by the o), this is extremely slow when you are used to 3G. Also at times its showing "Searching....." or "No Service".
You may find that while you are surfing you suddenly get this message "Safari cannot open the page because your iPhone is not connected to the Internet".
Hopefully this o2 mast will soon be up and working again soon as its very annoying when I'm paying for a service and getting a very poor connection like this.
Hope you found this information of use.