Hi ning , although these records during the 80’s were set without the penalties that are now in place . I have always believed that it has more to do with depth of a team than “corner cutting ” . A low 14 minute 5 k runner to 2.20 marathon runner was more than often put on one of the weaker legs. lap 3, lap 4 lap 6 and lap 7(due to being one of the slower runners in the team) . Anyone that was “caught by a race referee cutting over the centre line was automatically DQFed (no time penalties back then) .
To add to this it was almost impossible to cut a corner on lap 6 during an open year (cars were bumper to bumper ) – quite often athletes were forced into taking a longer route around traffic into the shingle on the side of the road. I’d like to add great runs by the first 2 teams and as you say especially Balchin . hopefully they all look at the records and aspire to break them. No reason why they can’t be beaten. Start planning now .