For Q3, you should write that you are asking the court to set aside the default judgment because you never received the claim form and therefore had no opportunity to defend. State that you are applying under CPR 13.2 and CPR 13.3. That wording covers both the mandatory set aside (if the court accepts the claim was never served) and the discretionary set aside (because you acted promptly and have a reasonable defence).
For Q4, you need to provide a short draft order showing what you want the judge to make. You do not need to overcomplicate this. Something like:
1. The default judgment dated [insert date] is set aside.
2. The defendant shall file and serve a defence within 14 days of the date of this order.
3. The claimant shall pay the defendant’s costs of this application.
Alongside the draft order you also need to attach your witness statement, which is your evidence. In that you explain that you never received the Letter of Claim or the claim form, that you only became aware of the case when you saw the default judgment dated 20 August, that you have had issues with unreliable post, and that you acted promptly once you found out. You can also explain that a Subject Access Request has been sent to the claimant’s solicitors asking for a copy of the Letter of Claim and proof of posting, and that you put them to strict proof of having properly served it. Make clear that even though you are waiting for their response, you are applying now to avoid any delay. You can also mention you hold a Blue Badge and therefore may have a valid defence under the Equality Act if the claim continues.
For Q9, you simply state that the application should be served on the claimant, which in practice will be via their legal representative DCB Legal. The court will take care of service, you do not need to serve it yourself.
In short:
Q3: Say you want the judgment set aside under CPR 13.2 and 13.3.
Q4: Provide the short draft order and attach your witness statement.
Q9: Write that the application should be served on the claimant through DCB Legal.
That covers the essentials the judge needs. Just show us what you have before you submit it so we can check it.