....Over the past month or so my lawn has changed. From being perfectly normal in appearance there are now areas where the actual blades of grass are about 2-3 mm wide.
Can anyone explain this and what I can do about it.
Sounds like some broad leaved meadow grasses have moved in. If its in patches, you could always dig out the areas affected and returf the areas with a finer turf or sow a fine lawn seed which does not include rye grass.
Keep well watered until its established.