In reply to MG:
> As is required by the HWC - if you are overtaken, you should leave room for the vehichle to pull in in front of you if needed.
You're probably thinking of:
168
Being overtaken. If a driver is trying to overtake you, maintain a steady course and speed, slowing down if necessary to let the vehicle pass. Never obstruct drivers who wish to pass. Speeding up or driving unpredictably while someone is overtaking you is dangerous. Drop back to maintain a two-second gap if someone overtakes and pulls into the gap in front of you.
That doesn't quite say what you suggest it does. It's made very clear in an earlier rule that it is the responsibility of the person overtaking to make sure that there is room for them to pull in afterwards:
162
Before overtaking you should make sure
∙ the road is sufficiently clear ahead
∙ road users are not beginning to overtake you
∙ there is a suitable gap in front of the road user you plan to overtake.
So what the Highway Code says is: you shouldn't
close the gap in front of you if someone
is overtaking, and if, having
been overtaken, you no longer have a safe gap in front of you then you should drop back. That's not the same as leaving room for the overtaking vehicle to pull in front of you "if needed" - it's entirely their responsibility to make sure there is room before they start the overtake.
If I'm stuck at the back of a queue on a windy road behind a FART and someone comes up behind me obviously keen to make a bit of progress then I will often drop back and give them room in front to make the pass. As you say, it's a courtesy (and it even sometimes seems to wake up the other bimblers in front to the fact that your vehcile won't explode in flames if you allow someone else to "get in front of you"). But the Highway Code doesn't actually suggest or require that you do it, apart from the general exhortation in rule 147 to "Be considerate", so if someone doesn't do it then they are being rather inconsiderate but they aren't actually contravening any specific rules.