A few points of clarification needed here.
Firstly, the service of the NIP. The prosecutors are required to post the paperwork to the address that the DVLA hold for the Registered Keeper (RK) so that in normal circumstances it will arrive within 14 days of the alleged offence (with the day following the allegation counting as day one). Unless you can show that you informed the DVLA of your change of address and that they failed to act on that information you will almost certainly fail in your bid to have the matter discontinued due to it being "out of time". In fact breach of the fourteen day rule does not automatically make the action invalid but the onus would be on the prosecutors to convince the court that they had a valid reason for not posting the paperwork in time. It is, after all, scarcely their fault if you could not be contacted at the address given.
Now, having received the NIP and request for driver's information, you are obliged to provide the information within 28 days. You need to do this regardless of how you decide to defend the speeding allegation. If you cannot or do not you will be summonsed under S172 of the Road Traffic Act for failing to provide details of the driver when requested. This is a more serious offence and carries a minimum of six penalty points. If you maintain that you do not know who was driving because of the "test drive" scenario you will need to plead not guilty to the S172 offence. You will face a trial and you will need to convince the court that you could not, "with due diligence" find out who it was.
Sentencing for speeding in the magistrates' court should follow their guidelines. The recommendations for a speed of 40mph in a 30 limit is a fine of half a week's net income (reduced by a third if you plead guilty) and three penalty points. You will also pay prosecution costs (about £85 if you plead guilty, about £350 if you are found guilty at trial). You will also pay a Victim Surcharge (10% of the fine subject to a £20 minimum).
I get the impression from your posts that you simply want to deny the charge without really thinking why. You said you are "99% sure you were not speeding". Are you saying you were not driving? Or are you saying you were driving but were not speeding? You need to think this through but most of all you need to return the completed request for driver's information (by November 25th if my quick calculations are correct). Also bear in mind that the current fixed penalty for speeding if £100 and three points. If you defend the matter and fail it will certainly cost you considerably more than that. If you engage a solicitor you will not get Legal Aid and even if you are successful the legal fees will certainly exceed £100.