Insufficient warm up for this type of session. It doesn't matter too much in training but if you get opened out properly before racing you will go even faster.
I though that might be the reason. It didn't seem to be a problem until a couple of years ago. Yesterday I ran the first 1,000m in 4mins 00secs and the last one in 3mins 35secs, so quite a difference. I'll try running (slowly) over from the house to the track before the next track session and see if that helps.