Just FYI, I looked up the issue of the CH and CHL radar networks being unable to pick up small raids of up to 4 aircraft.
It seems false.
There are many historical examples of the radar networks picking up individual aircraft (and some lone German recon or weather aircraft were engaged on that basis). Additionally, single, very slow friendyl aircraft were the standard system used for calibrating the radars.
In other words, as I hypothesized from the beginning, it is one thing that a small flight of four aircraft of Erprobungsgruppe 210 was not engaged by British fighters; it is entirely another issue to presume that that happened because the flight was not spotted by the British radar network.
There is the hypothesis that this flight was flying too low for the radar. That's not the standard practice of this unit, but of course it is theoretically possible - and nobody denies that under 100 feet, the radars couldn't work.
More likely explanations, all valid, are the following.
- The radar operators, or the air controller, made a mistake. Interceptions did occasionally fail for such a simple reason. The operator misjudged the height (a relatively common event especially at the beginning of the campaign), or the controller gave wrong directions.
- At that time there were two or three 100-plus raids on the table, too. Thus the 4-aircraft raid might have been masked by one of those, or it might have been correctly spotted, but ignored because of the more important targets.
- The raid was made up by 4 Bf 109s and/or Bf 110s; this identified it as a fighters-only raid, because of the speed, much higher than a bomber raid, and the British were under orders to ignore those.
- A combination of any/all of the above is a possibility, too.