Windscreen Issue

Have owned this car since Febraury 2011 and only recently noticed an annoying area on the windscreen which doesn't wipe properly. Worst in the dark but still a pain in daylight. It is right in field of view but I can't see any evidence of an earlier chip or crack repair.
Cleaned the screen with meths, oven cleaner, detergent and also used a new razor blade in a paint scraper. Nothing. The screen is as smooth as a baby's bum!
The other day when we got the latest temperature drop I had condensation on the inside of the screen. Started the engine, and put blower on full (expecting to wait the mandatory 10 mins for it to get up to temp) but immediately found a completely clear patch (on the inside) exactly where the outside issue was. This clear patch was completely round and about the same size as a tax disc.
Hate to say it - my daughter wondered if the screen had been repaired and there was something left on the screen! Never thought of that.
Has anyone else had this problem and how have they sorted it - without smashing the screen and claiming on the insurance?? Help please.

new wipers if on outside

hehe sounds like its inside out
TRY NEWS PAPER SOAKED IN VINEGAR

Thanks for response.
Have tried vinegar - forgot to mention that. New wipers may help - next on the list - but from memory - they only work on the outside!
Puzzled by the clear patch on the inside when all the rest of the screen is covered in condensation?

Thanks Wiltshire Landyman.
It appears that the screen may have been coated a while ago with something such as featured in your link. I work on ships and we have in the past used chemicals on the bridge windows - which are great for keeping the screens clear - until you use the windscreen wipers and it starts to ruin the coating making it much worse - much like I have at present.
Does anyone out ther use any of this stuff on their windscreens or just keep them clean and buy wiper blades when required as in the old days?