Well, why it should matter whether a code is logged or not is beyond me. It is not at all uncommon for certain transient faults to be cleared when they have not reccurred for some number of turn off/start up cycles.
The message you've received suggests that something is intermittently failing in the electronic throttle control. Why they wouldn't be looking at that is a mystery to me. A bad solder joint can cause weird intermittent issues like this (as can other things) but it's ridiculous that they're not digging into the electronic throttle control system just because there's no stored code. How I hate the fact that far too many service facilities depend strictly on whether or not there are active or historical OBD codes stored and refusing to act in any way if there are not! How many owners would ever even have seen the message you've seen and reported. That message is just as important as anything stored by OBD.
It's a tricky call Brian. Do you spend £5k on fitting new units? £5k on stripping everything down looking for a dry soldered joint? Spend a fortune just to find the fault is still there? A local dealer would regularly spend thousands of pounds doing that. A great way to make clients unhappy. Warranties pay for fixing faults. Not trying to fix them and failing.
Electronics are a nightmare!!
Are both your batteries in very good condition? Do you use a maintenance charger? Does it happen the day after you've taken it for a perfect drive or just some time after not been driven?
Have you driven it immediately for a diagnostic read once it does start WITHOUT turning the engine off before being read.
Are there ZERO fault codes stored? Induce a fault code and see if it gets stored. (short a pug lead to earth or pull a sensor off.
Off the top of my head . . . I think. . . . Earlier cars do not include the throttle control on the Mastercheck. It has it's own ECU and components on a completely different system. All the engine ECU sees is the throttle position sensor moving. If the throttle control is not motoring the throttle the engine ecu may just assume it's not being pressed. Having said that, that will not usually make the engine die (so I would expect a fault code for that) (but could do I guess).
You also used to be able to reset throttle position by turning the ignition on and pressing fully down and off on the throttle 5 or 6 times. Resets min and max settings. Not sure if that's true on yours.
It's very easy to criticise when somebody can't find an intermittent fault, but more than a good few times I've spent weeks using clients cars to pop out to do errands and never have a fault occur for me, only to give it back and it occur the next day/week. I'm very lucky to be rural and know I can use a car daily and safely without any problems around here. Workshops in cities do not have that luxury because parking and/or causing gridlock if a car FTP is a nightmare!