A couple of preliminaries:
There is no point in contacting the court. The matter has been listed for a hearing and will only be resolved by one. You will only speak to a court receptionist anyway who is not qualified to offer advice or authorised to take any action.
Were you issued with a warning letter or offered a fixed penalty to deal with this matter before the notice of court action? Is the notice you have received a “summons” (very unusual these days for motoring offences), a postal requisition or a Single Justice Procedure Notice? Does it give you the option to enter a plea (this is important)?
To the matter itself. So the less serious of the two alternatives, as I suspected. Having said that (in your circumstances at least) the more difficult to provide a defence. Section 144a says this:
"If a motor vehicle registered under the Vehicle Excise and Registration Act 1994 does not meet the insurance requirements, the person in whose name the vehicle is registered is guilty of an offence."
It then goes on to define what "meeting the insurance requirements" means. That should not concern us here unless your new policy covers "any vehicle owned by the policyholder". This is most unlikely as such policies are rare these days and even then you will have difficulty because the vehicle was no longer owned by you.
I think you realise that you are in a difficult position because, as far as the law goes, you were still the Registered Keeper. However, there is an "escape clause" which may be available to you. Section 144B provides exceptions to Section 144A. Among those exceptions is this:
144B (1) A person (“the registered keeper”) in whose name a vehicle which does not meet the insurance requirements is registered at any particular time (“ the relevant time ”) does not commit an offence under section 144A of this Act at that time if any of the following conditions are satisfied.
144B (2) Not relevant
144B (3) Not relevant
144B (4)The third condition is that the registered keeper—
(a) is not at the relevant time the person keeping the vehicle, and
(b) if previously he was the person keeping the vehicle, he has by the relevant time complied with any requirements under subsection (7)(a) below that he is required to have complied with by the relevant or any earlier time.
You were not the "person keeping the vehicle" (which is different to the "Registered Keeper") at the relevant time and you can easily show that. I believe (b) and the reference to Section 7a relates to your obligation to inform the DVLA that you had disposed of the vehicle which is where it gets tricky. The conclusion of Section 144B says this:
"A person accused of an offence under section 144A of this Act is not entitled to the benefit of an exception conferred by or under this section unless evidence is adduced that is sufficient to raise an issue with respect to that exception; but where evidence is so adduced it is for the prosecution to prove beyond reasonable doubt that the exception does not apply."
I believe that if you can produce evidence to show that you had complied with your obligation to inform the DVLA - even though you received no acknowledgement - it will be for the prosecution to prove, beyond reasonable doubt (a high bar to clear), that you did not. I'll check into this further and come back, but it may not be until tomorrow or Thursday.